Absolutely hilarious !! This is what the much vaunted "Special Relationship" now boils down to ! This is the kind of warning embassies make to nationals visiting conflict zones or countries run by dodgy despotic dictators (not even the UK qualifies as that yet). The fact is, Trump has made Americans everywhere more ashamed to be American... but even more importantly, more inclined to think they should be looking over their shoulders when they travel abroad (they should already be well practiced in that manoeuvre in the US when there's a police officer about).
Still, when States of Mind learned today that Trump praised Borat Johnson and is looking forward to meeting him in London we had to laugh. Two complete nincompoops trying to out-blonde each other, both have big tits and ass and are more than 50 kilos overweight, both with wives who detest them, and both preening narcissists who will bring their own hand-mirrors to the meeting. No doubt both will also speak the same time.
If I were an American, I'd keep a low profile too.
Answer: because Trump is trashing your brand, your flag, your values and reputation.... and your security.