AFL spokesman Eddie Vale puts it this way in an email. "Republicans are going to throw everything and the kitchen sink at this bill, including the chance they may offer amendments that we would normally support as a trap for Dems to try and kill the bill. So we will be letting people know that we will have their back if they vote FOR health care and AGAINST amendments, even ones we would normally want."
Now, there's a wrinkle in all this. Republicans can also change the reconciliation bill by challenging various provisions, arguing that they violate the rules of the reconciliation process. If they're successful, those measures will likely be stripped, meaning the language will change, and the bill will have to go back to the House for another vote anyhow. And if that happens, some progressives will no doubt ask: "As long as there's going to have to be another vote, why not throw a bunch of goodies for liberals in there while we're at it."
Looks like that's not gonna happen, though.