The answer is another question: Compared to what?
Whenever there is a vote, we pit the position of the current status quo against the position of the bill, as some alternative. (Sometimes the bill is pitted against something other than the status quo, which we call a reversion point, like a government shutdown if spending bills aren't passed in time, but that's not what we are discussing here). In 2010, that was the pre-ACA healthcare system against the post-ACA system. At the time, Snowe was sufficiently ambivalent about the choice that she voted to report the bill out of committee, but eventually voted against the bill on the floor. She preferred the pre-ACA system to the post-ACA system, by at least some small amount.
The pre-ACA system is now gone, unless the alternative the GOP chooses to introduce is a full repeal with no replace, through the nuclear option. If Collins were consistent, she would vote for that! Then again, that isn't the GOP's likely course of action, and that's part of why Collins has a credible threat to vote against the GOP's plans if they block Collins-Cassidy.
Obamacare is now the status quo. If that was at least marginally acceptable to Snowe, it is at least marginally acceptable to Collins since Snowe and Collins are basically part of the same hive mind. If the status quo is at least marginally acceptable to Collins, then she has no reason to let McConnell bully her into accepting anything dramatically different from Collins-Cassidy. Ted Cruz, Rand Paul, Mike Lee and the rest of the hardliners are mightily pissed off at the status quo, but Collins? Eh. She could be happier, but compared to letting Ted Cruz set policy? Collins really might prefer the current status quo.
Think of it on a line, from -1 to +1. Suppose the pre-ACA system was .2. Collins is 0. Suppose the ACA is -.25. Snowe initially couldn't tell whether the bill was closer to the Snowe/Collins ideal point or not because it was such a small difference, so she voted to report it out of committee, but after close consideration, they figured out that it was further from their ideal points, and voted no.
Now, here comes Ted Cruz, who wants things over around .85. If Collins and Cassidy get together and write a bill at .1, saying to everyone that it is .1 or Obamacare stays, can they do that? Well, if the alternative is to let Ted Cruz write something that moves things over to .85, then Collins really would prefer to keep Obamacare in place because -.25 is a hell of a lot closer to her ideal point than .85.
Of course, if the "replace" is to the left of the pre-ACA system, that changes things...
The key question is always this: compared to what?
