The issue of police brutality - how he was treated - is separate to whether United were within their rights to bump him off the flight. And the answer is...yes, in every possible way, they were.
And since they were, once he's asked to leave and doesn't he's now in the process of disrupting flight operations and interfering with a staff members duties by not leaving.
Very politely refusing to leave private property when asked to is very politely committing trespassing and that situation always escalates to involving the police.
Who, in the performance of their duties, should not use excessive force or cause physical injury but again - this is a separate matter to whether they have the authority to remove an uncooperative passenger, which as your link asserts repeatedly - they do.
And since they were, once he's asked to leave and doesn't he's now in the process of disrupting flight operations and interfering with a staff members duties by not leaving.
Very politely refusing to leave private property when asked to is very politely committing trespassing and that situation always escalates to involving the police.
Who, in the performance of their duties, should not use excessive force or cause physical injury but again - this is a separate matter to whether they have the authority to remove an uncooperative passenger, which as your link asserts repeatedly - they do.