It's vaguely titillating, I think. It's like I'm catching someone doing something secret - as most photographers try to remain unseen. Especially street photographers that beleive photos are ruined and devoid of human reality when the subject is aware of their presence. So by photographing street photographers I am voyeuristically capturing a secretive action.
On an unrelated note, I fucking love the structure in the MMoCA. The lines beg for photos as you can see in this.