I'm unsure if there is anything that technically prevents a pool owner from keeping a block, but doing so will only discourage people from mining with that pool (based upon a perception of bad luck) if they don't know about it, or encourage people to leave in droves if it becomes public.
Doesn't matter, IMO. A block's value is more than most small pool owners would ever make in fees during the lifespan of the site.
I've been in two small time pools that stole the first mined block & shut down their website. Not a big deal back then considering the block was worth just a few dozen bucks; Now the financial motive is $750 dollars for no work on your part beside hosting a half-assed site.
I'm not implying Sam is such a person, I'm saying the motivation is definitely there.
If not blatantly stealing & running away, then just taking the block & claiming you are undergoing an "unlucky round due to variance" and hoping the miners will just keep mining for you. You get 50 BTC and they'll never know about it.
Big site operators have zero motive for this because they'll make 50 BTC legit with their fees in the span of a normal operating day. They don't need foul play and will actually lose money in the long term if caught. Small pool ops have no reputation or business to lose. They'll make 2-5 BTC off pool fees in a few months if they're lucky.
This.
Especially considering that their pool structure is basically zero fee, Mr Sam wouldn't have been making much off of, say 50 GH/s of power. Even if he kept the 1% rather than redistributing it that's less than 10 BTC per month at current difficulty. Ignoring the variance problem completely, we should have solved between 3 and 4 blocks in this amount of time with our amount of power. that's 150 to 200 BTC worth of incentive - 15 to 20 months worth of profit at no real cost whatsoever.
It's a cool idea, but the incentive to cheat is too high and the pool owner's rep is too low. Maybe I'm overreacting, maybe Mr Sam is an honest guy and we're just having a really terrible round, but I'll be back over at Eligius for now, if for no other reason because at least I know Luke-Jr's rep better than Mr Sam's.