"The US ambassador had Belgian police stop our reporting"
Or reworded: "Belgian police stop our reporting simply because some foreign ambassador asked for it"
Or reworded: "Belgian police stop our reporting simply because some foreign ambassador asked for it"
If, indeed, the park was rented out for a private affair and the person managing that affair asked that someone be removed from the property, then like any case of trespass, it is within the purview of the police to remove that person.
It doesn't make the US look good, but I don't think it reflects poorly on the behavior of the Belgian police.
Local laws also matter.
In the US, getting someone removed from a public event without or with fabricated cause is likely to cause problems down the road, as the removed people might be likely to win lawsuits.
Belgium has probably - like the rest of the EUb - far weaker freedom of speech protections,
But IANAL, and just a second rate, third hand armchair commentator.