It is prejudice. end of. You can't ban a whole group of people because of the actions of a few.
If you do have some more serious issue with the Army, they are really just doing what they are told. If you don't like our foreign policy stop letting in the 50% of the people who voted Labour...
They are a private business and can choose to admit (or not) whomever they please - remember the story recently about larger women being barred from a nightclub?
That being said, it's still disgraceful, but they have broken no laws.