I think the outrage is stemming from a number of factors:
1) Statistically speaking, most PC gamers have an Nvidia GPU, and a portion of those are owners of RTX GPUs that can utilize DLSS, likely enthusiasts who have the financial means to buy newer, higher-end RTX GPUs.
2) DLSS does a better job than FSR at upscaling. Sure, some implementations of FSR can match DLSS, but it is definitely in the minority.
3) The spotlight on AAA PC titles being an unmitigated unoptimized mess blew up this year, so people naturally desire good methods to claw back performance as much as possible.
So, when you combine those three reasons, you get the following train of thought:
1) Dang. There's a bunch of AAA titles I want to play, but they are unoptimized as heck. I want higher frame rates without sacrificing visual quality.
2) DLSS lets me have higher frame rates without sacrificing visual quality.
3) I own an Nvidia RTX GPU. Therefore, my GPU supports DLSS.
4) Wait, this AAA title doesn't have DLSS?? How can I enjoy this game I paid $70 on then?!?
5) Oh, AMD was the one who sponsored this game, hence why it doesn't have DLSS. Screw AMD.
So people put the blame on AMD, rather than the root cause of the issue, which is unoptimized games. As many people have mentioned, game sponsorship has been around for ages; it's nothing new. Why make that the talking point now?