Why not do that in the post body from the start so nobody has to ~~whine~~ complain?

Personally, I for one would really appreciate it if you provided the archive link in the OP especially if you're posting paywalled articles. When the OP is the one who does it, then all the readers who want to follow the link that the OP posted won't have to, and the people who don't even know about archive sites will still get to read it when they wouldn't have otherwise (and maybe even learn about what archive sites are as a result). In that sense, it's not about entitlement, it's about one person doing it one time rather than (for example) 30 people all having to do it while others may not even be aware they can do it.

Another thing is that while it may be ultimately just a drop in the bucket, it does help cut down on the amount of traffic that mainstream western media sites will get, which I think you would agree is better than helping increase it for them. It's for that reason that just as a reader browsing, I almost always archive any MSM news articles I read, including the ones I see in posts here. On the occasion I do post, I always make a point to link the archive or frontend (like piped/invidious for youtube, redlib for reddit, used to do nitter for twitter, etc.) It's a very minor annoyance to have to do, as either the reader or as the poster, but I'm all the more appreciative of posters who do do it and wish that more would.

It's not like you have to, but it would be courteous and considerate if you did.