it is a political thing. Both the House and the Senate have to approve bills that fund these various government agencies, so if a president has at least one of

it is a political thing. Both the House and the Senate have to approve bills that fund these various government agencies, so if a president has at least one of the two backing him up (just enough that they can't override his veto) then he can say give me X or we won't pass the funding bill. It's standard practice to tack lots of little x things onto the bill because they wouldn't ever really pass on their own, but people don't want to deal with a shutdown to stop them. Sometimes that is a big enough issue that people will deal with a shutdown.