I have found it interesting that the concept of a “professional boarding barn” does seem much less prevalent in the US versus other places. There are “professional barns” – but as OP says, they typically have an in-house trainer and the expectation of being in that program with that trainer.
When I was in NL and Belgium, I did not find it all that difficult to find professional boarding barns. Prices were transparent and public, services available were made very clear, there was no “tailoring” or favors. Barns may have had communities develop within their boarders but they did not serve as a community (if that distinction makes sense). Both countries also tended to be MUCH more rule abiding in general as a culture. If the barn rule was dogs were leashed within the barn but could be off-leash in fields / open areas of the property - that tended to be respected religiously and by everybody. If someone does not adhere, they get a message (or in person conversation if convenient) that reminds them of the rules and asks that they respect it.
Here in the US I have been fortunate to have generally positive boarding experiences, but it’s MUCH more of a relationship-driven, ad-hoc cases treated at the discretion of the BO / BM. I tend to see high frequency of the “unleashed dog” scenario either a) going un-addressed or implicitly approved by BO / BM because that person is a friend / special exception OR b) BO / BM may be disgruntled by it, boarders may vent about it to one another, but nobody actually brings it forward to the transgressor in a professional and simple manner. There’s clique-y politics without just simple professional / business-like confrontation about it.
This isn’t true for ALL boarding-only barns (i.e. no program - I can’t speak to barns with strict programs as I’ve never used one) in the US nor is EVERY barn in BE / NL a perfect image of professional behavior. But in my experience that seems to be an on-average difference.