I love street art. But I understand your feeling. Sometimes we don't know who made it. But In my point of view it's okay to share. It the artist doesn't want that, I guess he would make paintings instead of street art. Last week the Urban Art Festival 'Museo del Mar Menor' started in my neighbor town in Spain (I'm in Belgium at the moment). The town is famous for the street art (more than 180) and I can't wait to be back and explore the new ones.