JustAFan
The Adebayo Akinfenwa of football photoshoppers
They've never met Dany and have been told there will be a way to stop the dragons. The promise of an upgrade in status of their houses helps also.The taking of Highgarden did seem a bit suspect...I can imagine the castle being under-provisioned or undermanned considering Olenna's focus has pretty much purely been on revenge as opposed to tactics etc, but the fact that it seems like all of the Reach lords have supported the Lannisters in spite of the fact Daenerys has three dragons and (did) have the advantage seems a bit of a stretch.
Plus the fear of the dirty foreign dothraki and unsullied.
Obviously just saying oh she has dragons we all surrender to Dany would be a pretty lame story line.