Leeching is an entirely different subject.
On whether or not it is ok to tell others how you think they should play, on this we can agree to disagree here. To me, context is irrelevant - it is simply a matter of personal preference. I'll let others play as they like so long as they are not willfully and deliberately trying to grief me or everyone else.
I can adapt and let people enjoy playing the game the way they want to because that is the purpose - for everyone to have fun.
KB does not bother me in the least. If I'm on my tank and someone knocks a mob away with KB, I do my job and move to re-acquire aggro or wait for them to come back to me for more. It keeps me on my toes. If I'm scrapping, I'll switch to another target or chase one down and pummel it until dead and then move on to the next victim. If I'm on my blaster, I'll just choose different targets until the ones knocked away come back into range. Same goes for my controllers, defenders, corruptors or whatever other AT I may be playing. I'll adapt to the flow of what the team is doing.
At least to me, if someone is playing a way you don't care for, then find a different team or solo. Change yourself instead of trying to change others.