It's a lazy solution to a problem that isn't a problem.
Blizzard seems to have a hard on for stopping split raiding and even openly asked the community around the launch of Legion for ideas to curb its prevalence. Forced PL was one of the proposed "solutions" and it was debated with exactly the same rebuttals this thread has produced for the last 20-something pages.
Ultimately, the consensus was that it was generally a bad idea and the people it would most negatively affect -- Mythic raiding guilds -- represented such a small fraction of the playerbase that the need for loot control on a larger scale for everybody outweighed their desire to stop guilds from gaming the system.
But alas, we were wrong. Forced PL is now a thing and we're just going to have to deal with it. Why bother trying to find real solutions to problems when you can slap a fucking band aid on it and wash your hands of it? Split raiding has effectively been "solved" and the message is clear:
"Just keep grinding our stupid fucking game. We don't give a shit what you do in it as long as it keeps you subbed. Oh, and make sure you buy the next pretty pony we put up in the lootshop. "
The minority is essentially told to fuck off and deal with it while the far greater number of casual players are none the wiser as they likely never really gave a shit about ML to begin with (and, as this thread has heavily proven... mostly perceived it to be unfair anyway). Since the latter is the only demographic Blizzard cares about these days, lazy decisions like this will continue to occur.