Romney could have still waited on the legislature. He didn’t have to do what he did.
Sure, he could have gone rogue and ignored the Supreme Court's decision, in just the same way Obama is choosing to ignore laws he doesn't like.
I know many conservatives argue just that. But if one supports the point of view that the Executive Branch should selectively enforce laws for political reasons, then basically the Rule of Law becomes irrelevant, and will eventually give way to anarchy.