Well, how could we possibly enforce that? People are free to discuss OpenStreetMap wherever we like - we can’t prowl round physical and virtual places and put a physical or virtual bag over someone’s head if they start talking about OSM in an “unauthorised location”. That said, it’s a shame when a group chooses a platform that isn’t open to everyone (such as one of the US corporate giants that may be on the wrong side of a firewall in other parts of the world).
The OSMF has a Commitment to Open Communication Channels, which means that when the board or a working group say anything publicly it can’t only be said on e.g. Slack/Loomio/etc. - it needs to be said where everyone can see it. It might also be said on Slack (if I was saying something that primarily was aimed at the US community it’d be daft to ignore the place that most of them congregate).
However, no-one is forcing any OSM user to use a non-free channel. Although it’s probably the most popular channel, plenty of US-based mappers don’t use Slack. In the UK the Loomio group that OSMUK use seems to be relatively inactive compared with e.g. the mailing list. That doesn’t mean that Loomio isn’t good for what the OSMUK people want to use it for; just that there are all sorts of mappers in all sorts of places and one size does not fit all.