Spats' Stab in the Dark: I wonder if the walk-off is to prevent a vote at all. I'm entirely ignorant about the laws on how the Iowa legislature works, but if a quorum is required even to hold a vote, and the walkout drops attendance below a quorum, then the departing legislators might prevent the majority from being able to vote on the measures they'd like to pass. In that case, it's not "taking their toys and playing elsewhere," it's successfully blocking legislation. Just a thought.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.