Of course it is. Defending allies, our economic interests, and shared strategic resources is more than just OK, it's one of the most fundamental reasons we have a government and a military in the first place.
Saudi Arabia (ugly flaws and all) is a US ally and we have defense agreements with them. You're angry that we're not defending the Kurds against Turkey (another ally and a NATO member too), but a small deployment of mostly air defense assets to an ally that just got bombed by Iran (a mutual adversary) is somehow terrible, because oil?
If you wish, argue that we're on the edge of some kind of post-petroleum green revolution and that oil isn't important any more. You'd be wrong, but there's a discussion to be had. Or argue that it's strategically unwise to do whatever it is you think we're doing in Saudi Arabia.
But don't argue that defending the Kurds is right because they're allies, but defending the Saudis is wrong because they're allies (who have oil).
I think we have a whole bunch of competing points which are clouding the ultimate issue at hand, and Trump's underlying reasoning (or lack thereof) is making things even more confusing.
Firstly, what is the ideology from which we are approaching stationing our military overseas, either in Syria or KSA? If we are talking purely from a Randian isolationism standpoint where the rhetoric right now seems to be that bringing our troops home from the middle east/keeping them out of harm's way is the most important priority, then it would strike me as strange that sanctioning relatively unnecessary troops in KSA would be a desirable action. We have an enormous amount of oil in the Bakken, Permian, Gulf, SPR, NPRA etc, and there are plenty of other countries dependent on middle east oil who have standing militaries who can send their own troops in a support capacity against Iran.
If we are approaching the issue from more of a what strategy overall makes America "safer," or at the very least best serves America's overall geopolitical interests, then it would be appear that neither a sudden withdrawal from Syria nor denying a small deployment to KSA are desirable. I think you made a point earlier that the Kurds are allies but not NATO allies, and that they are also a non-state actor. I'm not sure how relevant that is considering the bulk of our counterterrorism intelligence and ops are against non-state actors, aka ISIS, and the Kurds have been extremely valuable in suppressing ISIS' growth. Additionally, I think we will soon lose count of the hundreds if not thousands of ISIS prisoners who will go free inside Syria and make their way towards our allies in Western Europe. God help me, but I have to agree a little bit with the point that Liz Cheney made in that having a geographic distance of 7000 miles does not necessarily protect us from a small, well-funded, and determined enemy. Furthermore, it is almost a 100% certainty that we will need to rely on both state and non-state allies for our future counterterrorism operations or operations against adversaries such as NK, Russia, Iran, China etc, so what kind of message does it send to them when we use them and then lose them? Does that make us more safe?
In regard to Trump, he adds confusion to the issue because he is so heavily conflicted. It is difficult for us who are interested in his actual motivations to truly believe that he siding with Rand Paul purely out of ideological agreement. Indeed, I think it would be foolish to do so because Trump has such large business interests with Turkey (and KSA / MBS). How do I know that he wants to withdraw from Syria because he cares about the well-being of our soldiers, or because he wants to ensure that
Erdogan doesn't pull his royalties from Trump Tower Istanbul like he almost did in 2016?