There definitely are those who are doing it.
There was this guy who was live broadcasting how he is doing it ah how easy it is to do, he struggled with the live streams , the platforms were banning him.

i don't know of any "laws" that would enforce such a thing. this is a free market and what we call "51% attack" is a feature of proof of work algorithm. so in the end they wouldn't exactly be doing anything illegal if they roll back a bunch of blocks.
the only illegal thing could be if they had some legal contract to make a payment through that altcoin and they reversed THAT transaction. otherwise the rest of the transactions aren't "reversed" they are just now in another block and the block reward that went to the other miner is now invalid.
BCH did a 51% attack on their own chain recently and nothing happened! they literary rolled back a block that was mined by an unknown miner that wasn't in their centralized team and stole their rewards.

they aren't ASIC proof because there is no such thing as "ASIC proof", it is an illusion created by a bunch of delusional developers who were trying to sell their altcoin. what we have is "resistant to bitcoin ASICs" or in simple terms coins that can not be mined with SHA256 but are mined with something else like scrypt, sha3,... and ASICs can be created for those too.
to 51% attack these coins, specially since many of them have little hashrate, all you need is a bunch of GPU rigs. like what happened to BTG.

Two reasons:

1. Not financially worthwhile to commit resources to destroy something when that resource can be better spent mining BTC.
2. You can't "destroy" things in crypto.  A 51% attack will just result in a hardfork/rollback.  Your resources are wasted and their legacy crypto continues as previously intended.

Hardly anyone is mining altcoins now, as their value have crashed by as much as 50% during the past two months. Most of the altcoin miners have shifted to coins such as Bitcoin and Bitcoin Cash. If you check the mining difficulty and hash rates for the established altcoins (such as ETH, ADA and LTC), many of them are showing sharp declines.

"ASIC Proof" …. " ASIC resistant" … same thing.    The crux of the matter is, SHA256 miners will not find it easy to mine these coins. I know some of these miners might still have some GPU's that they use to mine Alt coins, so it might be easier for them to launch such an attack, if they concentrate their efforts on an small Alt coin with Scrypt algorithm.

The main thing is that a larger Pool can easily destroy Alt coins if they wanted to do that, but they are not doing it for some reason. The obvious answer is that the gain is not worth the effort to do this.     

Number 1 and 6 are probably the best reasons.
 It makes me wonder if the 51% attackers can be identified or does the identification depends on coin that is attacked?
Are the locations of all miners known? I guess mining pools are known by their locations, ip addresses and even names, right?

The main reason they don't do that is because they don't want to waste their time and hash power. Because they continue to compete with many Bitcoin miners around the world to break the Bitcoin block. Because Bitcoin is the most difficult coin to mine until now. So why waste time harming Altcoin if you can make a big profit with Bitcoin.

Well, is there a need to spend our time and effort in destroying these coins? Check the charts for the last 2-3 months. They are destroying themselves. Established alts are down by anywhere between 30% and 40%, while the smaller shitcoins are down by 50% or more. Why we should waste our resources in destroying them, when they themselves are doing a very good job with that?

If we use a 51% attack, these shitcoins will just claim that Bitcoiners were afraid of the competition from altcoins and that's why they commenced the attack. Is there a need to give them this false victimhood? I would rather leave them alone to their own fate. Ultimately they are going to die out, irrespective of whether the Bitcoiners are going to act against them or not.

Although that might mean the coin survives, it's irrelevant to the attacker. Rallying a community and potentially hard-forking would take days to weeks. The attacker only needs a few hours to sell their bags.

Let's say an attacker buys $10,000 of a coin. They transfer all that coin to Exchange X, and immediately dump it for BTC. As soon as it is sold, they withdrawal the BTC and 51% attack the altcoin to reverse their deposit. They then transfer all of their returned coin to Exchange Y (or even to a new account on Exchange X if the order books are big enough), and immediately dump it for BTC and withdraw. They could potentially repeat this process several times depending on how quickly the exchanges act to freeze the coin being attacked. They could potentially make more of a profit by staying off of exchanges altogether and setting up several private trades, and executing them one by one in succession with the same coins.

Sure, after this is all done, the community can hard fork and try to mitigate the damage, but the attacker is already long gone with their BTC. Having said all that, given we produce 75 new BTC an hour (on average) from honest mining, I don't think it is worth it.