It's maybe a bit preemptive, but with several US states trying to pass bathroom bills, I don't know if it's an overreaction to warn people who might not be paying attention to international news. I don't know if every person in the UK would be aware that different states have different laws. It's likely that the last LGBT thing they heard of happening in the USA might be the equal marriage ruling, which would maybe give an overly positive view.
People should definitely research places they're going to visit before going... but the foreign office might be their first stop in that research