The situation goes back a very long way into the common law determination of what a charity is - far further than the separation of the USA and the UK. One of the classic heads (there has been recent statutory interference in the UK) of "charity" is religion. A charity is only a charity if everything it does is charitable. As soon as it does ANTYHING that is not, the charity is no longer a charity (and so is no longer tax exempt, and incidentally is no longer free from the rule against perpetuities - a long story there).

That is why charities set up their trading arms as corporations that are not charitable.

Dead easy. Say what you like. But if your words or acts are political, you are not a charity any more. It's a tightrope walked by far worthier oranisations, like Amnesty International.