These coins that are listed doesn't seem to be in demand to begin with, and it's just another of the company to actually pull off some resources they have spent to keep the market for these coins live. Also, exchanges have the right to delist a coin which they see unfit to be in their exchange because they are getting profits from trades that happen within their platform. If there is no demand, no profit, and you know the rest.
Exactly, there was no real demand on these coins, as far as I have observed.
I can proove that with one very important thing: after the poloniex have announced that they are going to de-list all these 17 altcoins, the price have simply broken.
People were panic selling, basically most of the volume on the poloniex was being dumped, or in a good path to be dumped.
It is kind of funny, because some people are saying that poloniex have cheated their customers: but you need to remember that they would cheat, if these coins would pretty much dissapear, without doing de-list from the polo side.