A value is also, and not to a small part, generated by the group admins and moderators - which is also why there have been a fair share of controversies regarding subreddits whose mods sold out (e.g. r/wallstreetbets).

Fostering a community is hard work... kicking out spammers, fraudsters and peddlers of illegal material (cp and DMCA), dealing with trolls and shills, directing newbies to useful resources for example. Without moderation (or too lax moderation criteria) your forum will end up overrun with legally questionable material (4chan, 8chan, 8kun), alt-right conspiracy theories (Parler, gab.ai), porn (r/worldpolitics) or questionable "advice" (r/legaladvice).

The problem is that moderating content is rarely positively rewarded - HN with the constant praise of u/dang for their moderation work is the example proving the rule. You're too lax? Users complain that the community is "devolving" from its purpose. You're too strict? People will raise off-site shitstorms or run organized brigading attacks.

This is why group admins deserve to be compensated for their efforts (and in the case of FB mods, way more compensation given the constant onslaught of cp, gore and other extremely vile stuff).

Why should Facebook get paid when the value is generated by the community?

They manage the community, if you have unmoderated groups you get 4chan and similar which no-one wants to advertise on.

Also it incentivises moderating groups rather than just letting them be and doing the bare minimum.

I still don't like the idea but entirely for the reason that it adds financial incentives to demand moderators force groups to be advertiser friendly in more ways than just removing toxic content. If advertisers don't like something it will simply be neglected in comparison to the more advertising relevant ideas.