Advertising in public space is legal (with some minor restrictions). Municipalities even sell out public amenities such as toilets, bins, bus stops, etc. to commercial exploitation through advertising agencies. As citizens, we are every day, without our consent, hit with dozens of posters telling us to buy, what to buy, and not to be satisfied before we have bought it. When the public does not have any more needs, they have to be created in order to make the wheels of late capitalism to keep up the pace. Completely ignoring the obvious fact that we cannot keep growing forever, sooner or later (rather sooner, if not already there) we will reach the limits of this limited planets recourses. Still big time advertising is legal and promoted by public officials.
On the other side street art is barely tolerated on a very few designated walls. Why is something legal as soon as you can make money out of it, and illegal when free of charge, only trying to win a smile, some wonder and a new thought?