We don't, obviously. And in most cases positions on bills like this are a function of geography more than politics -- if a bill is desired by local lobbyists the local representatives are going to support it. This is the problem with single-issue voting in a two party system; it simply doesn't work the way you want it to except in very limited ways:

SOPA died because its proponents wanted to avoid the controversy that was growing over it and dropped it, seeing litle upside. That is the way single-issue politics works in the US system. And the system worked, and the bill isn't law. Trying to prolong the fight at the ballot box does nothing more than make you enemies among the supporters of basically every other single-issue subject in american politics.

A really important related point is that not all issues are like this. A vote at the ballot box isn't going to do anything about SOPA or follow-on bills, but it is likely to have effects on future tax and health care policy, court appointments (hint: which party's judicial appointments are most friendly to heavy-handed internet regulation?), and (in the executive branch) the choice to engage in wars of aggression.

Don't fool yourself into being a single-issue voter. This isn't a parliamentary system.