The average Joe is entitled to state their opinion, in places such as here, because they don't have more influence than another poster arguing a different viewpoint. That's part of a debate. Although even then those outside the UK should be careful since the same, 'not your concern', accusation could be levelled at them; depending on what was written.
However someone holding the office of head of the most powerful powerful nation in the world is not simply your 'guy in the street'. Whether it influences us personally, or whether we pay no heed, is not the issue. There are a lot of undecided out, there a portion of which may conclude that they can't see the best option and instead of abstaining decides, "let's go with what the 'important guy' said"; he must know something.
This unfairly influences the future outcome of a country that is not the legitimate domain of the 'important guy'. There may be a decision that would benefits him and his country, but that should not effect the result of the UK citizens' decision. Therefore the 'important guy' should keep their opinions to themself.
We're not discussing, just, "I'm not keen on this guy or admire that", here; we are trying to convince fellow UK citizens the the point of view we hold is the correct one for the future of the UK and future generations. It's a totally different ballgame.