I tend to to think of apps like Photoshop, Textmate, and Transmit and think "A recurring payment? No way." And yet somehow when I think of 37Signals, it doesn't

I tend to to think of apps like Photoshop, Textmate, and Transmit and think "A recurring payment? No way." And yet somehow when I think of 37Signals, it doesn't seem so bad, but I can't put into words why one makes sense and not the other Simple: 37Signals has server costs on an ongoing basis to maintain a customer - Textmate/Transmit/PS do not. Most consumers understand it's a hosted solution and are ok with paying a recurring payment. If your solution isn't hosted and it's a onetime download kind of thing then it would only make sense to charge them the one time.