You're right, but that's not necessarily a bad thing. The alternative to NASA being used as a source of pork in budget negotiations isn't an efficient allocation of those funds for NASA science missions minus waste. Instead, the most likely alternative is likely a much smaller NASA or even
no NASA, period.
Americans are proud of NASA, but in more of a broad, general sense than anything concrete. That pride doesn't translate very well into increased monetary support. On the contrary, even though every dollar is spent "here on Earth" (there are no orbital superstores, after all), there's still a pervasive attitude that the money is being sent "elsewhere." It's always been this way; just weeks after Apollo 11, a Newsweek poll showed that "56% [of 1,321 Americans with household incomes ranging from $5,000 to $15,000 a year] think the government should be spending less money on space exploration, and only 10% think the government should be spending more money." That was at the height of moon fever, when Americans were giddy with a sense of accomplishment. Give voters a binary choice, and space funding will always lose.
For congress, think of NASA as a big bag of money they'd love to play with. The public sort-of loves NASA enough to give it a thin safety margin, to the extent that no politician wants to be the heartless bastard who snuffed out everybody's childhood dreams, but not enough to give it much additional support. Many members of congress of even agree with NASA's importance in principle. NASA is a big, bold example of American accomplishment and ingenuity. But they've all got their own ideas on how some of NASA's funding could be used elsewhere.
The "pork" aspect, even if it means continuing with SLS development despite not having a clear purpose and mission for it, gives NASA a base of political support that it'd otherwise lack. It might mean less science as a result, but that's not the bargain being offered to NASA. They certainly wouldn't get more money if they could eliminate the pork and other inefficiencies forced on them.
STS and the Space Shuttle were a perfect example of the kind of faustian bargains NASA had to accept. Originally, NASA had no post-Apollo plans because they didn't want to lose focus. It wasn't until 1968 that things changed. Eventually, the post-Apollo plans were limited to STS, and even then, it was the promise of the Space Shuttle being able to perform national-security missions that helped it squeak by. That promise led to some pretty significant design compromises NASA didn't want, but they didn't have any other option. The alternative was no shuttle, not a better shuttle.
NASA exists within a very particular political and budgetary paradigm that is not friendly to it. It's easy to imagine a better NASA. Regrettably, that's not a NASA that's on the table. Instead, we have NASA as it is. It could be better, but it's accomplished a hell of a lot while managing to survive.
If you're at all interested in NASA's early battles after Apollo, I'll recommend (yet again) John Logsdon's books. They're an incredible insight into the challenges the public doesn't really see, and helped me rethink some of my earlier antagonism towards the Space Shuttle and aspects of NASA's manned space program.