Welcome to the world of crypto. Since there are only few trading platform that are available in the market, there is nothing we can do but to use it. Not all altcoins are available in every platform, so boycott is not an option unless there are other options. It happened already on Poloneix. Base on history Poloneix dominates the trading volumes for years until Binance appear. Wait more years and you will have more options and traders will be a winner because of trading platform competition.
Poloniex is government licensed, stocks.exchange isn't. Both are in USA. Sending your coins to any institution should be deeply cautious, but the operators of stocks.exchange messed up big time and absolutely need to be looked into by the Feds.