The Simplify site has a wonderfully simple claim of "it's legal", which I can't help thinking is a over simplification outside of the US. Expect legal cases in various countries if this becomes popular.
Do you suspect that streaming music from your home computer to your portable computer would be illegal for some reason? Maybe you're referring to the "other people" part of it, but it's clear that they are referring to a very small group, and if that's illegal where you live, it's easy enough not to do it, right?