Sia stores files in encrypted form, so there would be no way to know what kind of files someone is storing and whether or not they are illegal.
People store illegal files on Dropbox too, so it's really no different. People use the internet for illegal things all the time, we don't go around banning the entire internet because of that.
As for the Sia price going down, well before that it went up 500% in just one month. Just a temporary retracement. What goes up must come down.
Looking forward to it.. this makes a lot of sense. I'm excited to see how high they can fly ;D