That's like saying putting something on a scale isn't an objective measure of its weight because you can just take the batteries out of the scale. I'm considering it objective because the more correct answers you get, the higher your score.

We can argue about whether or not the stuff on the test is important. We can discuss the various environmental, cultural, and socioeconomic factors that go into the kids taking it and how that impacts scores statistically. We can even talk about the various medical and neurological factors that go into test taking generally and standardized test taking specifically that can affect some people more than others. I'll even concede that the SAT/ACT isn't a test of your intelligence so much as it is a test of how well you did on the SAT/ACT, just like putting things on a scale may at best be a measure of its weight relative to other things on that specific scale.

But at the end of the day, more right answers === a higher score, and I'm not sure how you get any more objective than that when it comes to something as fluid as academic ability.

Exactly right. I think the critique is that it serves little value in college admissions because aptitude at taking the test is not an indicator of anything else. But IMHO, defining a difficult task and judging people on their ability to prepare for and execute on that difficult task is a useful means of measuring whether that individual is capable of doing that very thing when facing other difficult tasks, as they certainly will at college and in life. That alone has value and relevance in college admissions.