But to the constituents that put these GOP reps in office, it IS voting on it.
Also, I think there are two very specific reasons they're doing this, and they cut to the core of the issue. First, republicans have been talking for a few years now about how awful the ACA is. It's been the go-to-dead-horse for the GOP. Anything bad happens they can tag it to Obamacare. "Oh, unemployment is up? Obamacare! People are on foodstamps? Obamacare. Benghazi? OBAMACARE" etc.
Through all this, they've made some incredible claims about Obamacare and they've made some incredible claims about Obama. When the ACA is implemented, they don't have that punching bag anymore. This leads into the second reason, and that reason is they're scared to death the ACA will actually work.
If the ACA doesn't do all the crazy shit the republicans have said, they will look TERRIBLE. Further, if the ACA works as it's supposed to and people like it, it could literally doom the careers of the same people who've been screaming it's the worst thing to ever happen to America. They will be shown to be uninformed hate mongers who spread fear for nothing more than political gain.
How's that different than any other time most politicians speak? I think it will be different because it will be the first time that I could think of where a wide swath of politicians would be put on blast as completely inept. We've had one offs in the past, people like Clay Aiken with the "Legitimate rape" comment, Michelle Bachman saying Obamacare was literally killing people, and that one republican saying that 93% of what Planned Parenthood did was abortions (only to walk it back and say "I did not intend my statement to be factual"). So we get those crazies by themselves, but can you imagine the kind of damage the GOP as a WHOLE will take if the ACA works? There will be literally MILLIONS of voters who will look at the tactics the GOP used and never vote republican for the rest of their lives. And the GOP knows this, and I think its why they're willing to crash they economy. They're fighting obsolescence.