This is not a "these days" thing. Most rights essentially being abrogated at the border is a thing we've had for many decades now. In particular, no non-citizen has the absolute right of entry, and border agents have discretion bordering on insane when it comes to not admitting people or temporarily detaining them. Worse yet, this being the case, the job attracts the kind of people who relish the petty power trip. I've seen this many times back in 2010-2011 when I was traveling regularly as a non-citizen between US and Canada.
What gives US americans the right to deny anyone access to the land they stole from the indigenous they genocided? Why do people think they are entitled to take other peoples land and then prevent others from coming too?
Up in the air. SCOTUS didn't decide on this specific right. They did rule on other constitutional protections though, sometimes granting, sometimes denying them to non-citizens. It also depends if you're just arriving or already are in the US. For example, illegal immigrants who jumped the border fence have more rights than legal immigrants who didn't leave the airport yet.