The difference being that meatspace activism is almost pointless these days. It might get a 30 second mention on the news on a slow day, but otherwise you're just shunted into a "free speech zone", traffic gets routed around the protest and is flat out ignored.
Hacktivism on the other hand, has relatively immediate, noticeable (sometimes very much so) consequences that can either cost an organization money or if nothing else cause embarrassment.
Meatspace protests make you feel good, and are probably amusing to the powers that be. Online, a few people can a real nuisance, which is what activism is trying to do: be a nuisance until a change happens. [sarcasm] We can't have things like that happening in this country. Obviously we have to set an example for these 14 people. [/sarcasm]