There's a game of brinksmanship going on. The proponents of the conference report version have options too - for example the 3 month extension in S.2082, which is not contentious at all. Frist asserts that a 3 month extension is unacceptable, so won't bring it up for debate or vote even though he can. Frist asserts thatteh President, given a choice between a 3 month extension and sunset, would choose sunset.
Supremely ironic, given how the opponents of the President are often the ones accused of being unappeasable.