I am american. I live in Scotland (North Ayrshire). I have been here for 18 years. I sailed out of Harwich a couple years ago and found the British population onboard the ship very overwhelming. I sucked it up because, after all, I am in their country. I would never put down and call names to the people of the host nation. That is just bad diplomacy.
Regarding the "Welcome home all Americans" banner: it was in America, in the Nation's Capital of America. If they can't say it, then there is something wrong with the country. I am retired US Navy, and I am often appalled at the way the UK treats their service members, both active and veterans. (I know that All Americans does not mean servicemen and women, but just go with me on this). Open your mind and see how other countries act and see if it can be used in your own country. Maybe if everyone did that, it would be a better world.
Remember, it is an American company, sailing from America. It is going to cater towards the majority of their guests from that location, Americans. I've heard that cruises from Galveston are heavy on the Country music. They're Texans after all.