He doesn't need any latitude with the tea party people anymore. Him announcing he'll step down takes away all the power the Tea Partiers had over him.
If he just hates the Tea Partiers, wouldn't the biggest FU to them be passing a full spending bill, not a CR?
Even if he's completely checked out, and is only sticking around to put whatever gets handed to him on floor, wouldn't McConnel work night and day to create a full spending bill that he'll send to Boehner to get passed?
I think pigeon's basically right that this is about keeping a lid on the Tea Party until an establishment candidate can clinch the nomination. And in general Boehner has been Speaking month-to-month for years now; he's almost always just concerned with getting through the current crisis. He keeps this from blowing up by stepping down and passing a CR, where his stepping down adds credibility to his usual "next time we'll fight for real" promise. Anything that puts off another showdown until after the 2016 election is going to come across as the opposite - it's Boehner performing one last act of sabotage to undermine the noble conservative cause before leaving for a lobbying gig. That's not good for the establishment candidates at all - it supports the narrative that no matter what they say you can't trust insiders to uphold conservative principles and all that.
Edit: The establishment's main argument is that they want exactly what the Tea Partiers want, but that they know how to do politics in order to get it. They can get away with a tactical retreat if it looks like part of a larger plan, and this has basically been Boehner's approach for years. That's a very hard sell when you're proposing that Republicans not fight until after Hillary beats Jeb.