I'd assume it's because AC wants some level of revenue if they're going to let you in "for free".

How do you even get a UC one-time pass without paying $50? There's one(?) credit card that gives you two per year? Any other ways?

Everything else is paid memberships.

AC gives every SE 4 transferrable MLL passes. Lower tiers get fewer. I think you might be able to select more as a benefit at the lower tiers as well. There are a lot of "free" MLL passes floating around.

I appreciate it's confusing when you book AC 5109 SFO-YVR on AC.com and it turns out you can't use the MLL pass, but I don't think this rule is unreasonable or inconsistent.

If I go to AC.com and spend $10k for J on AC 9104 YYZ-FRA tomorrow, I can't get into the Signature Suite.
If I go to LH.com and spend $10k for J on LH 6789 YYZ-FRA tomorrow, I can't get into the Welcome Lounge.