I don't agree with a ban on marijuana, but I see it more as a test of whether you can follow the rules. If you are brazen enough to ingest THC, knowing it's banned but also that it stays in your blood for weeks, you might have a tendency to use actual PEDs.
It's banned in-competition which seems to make sense as you wouldn't wanted someone competing in mass start events while stoned. There's apparently some evidence it's performance enhancing from animal studies and athlete interviews, but that sounds like it's a bit up in the air. With it being decriminalised across the US and in other jurisdictions, it seems like something that should be moved to the watch list and dealt with in a similar way to alcohol (which was removed from the Prohibited List in 2018).