Mixes get kicked off of SoundCloud because they have a higher chance of one of the X songs/samples in them being picked up in the copyright detector net. The fact that it doesn't happen, or that it happens less, on MixCloud only goes to show that MixCloud isn't playing ball with copyright holders, or hasn't been approached to do so. They don't have some magical blanket license to let anyone, anywhere, for free, upload a mix with copyrighted material.