Companies
really like things that limit their liability. SOPA/PIPA is one of the largest, federally-mandated wavers of liability we're likely to see in the next little bit.
These companies want to process payments and collect their fees. The transactions in question are minuscule compared to their overall market. Someone comes along and tells them: Just do it this way, and you'll never end up in court. What do you think they are going to do/support?
(Plus, they / senior management are "in bed with" and/or effectively the same as senior management of many of the other SOPA/PIPA supporters.)