@Michael Schneider.... on the contrary, you seem to believe that the US has "more diversity", but other than in a few clusters scattered around the main cities in the country, the US isn't as diverse as many cities here in Europe. Madrid, for example, has over 20% of its population from over 110 different nationalities. In Barcelona, my home city, only 51% of its residents were born here. This is to say that we are quite aware about "diversity".
However, as my Danish colleague @Morten said earlier, the main issue here is the different approach to racial issues on both sides of the Atlantic... while racism is still present, in a degree or another, in most countries around the world -including yours and mine!-, the prevalence of this "illness" (allow me to label it so), seems to be much lower in our shores these days and, as mentioned earlier, the context in which you say something or use something --which on its own could be offensive-- does matter.
Welcome to the great world of idiosyncrasy, my friends! Not every culture sees things the same way: history, cultural backgrounds, etc. make things one way in one country and another in a different country. Without willing to start a new debate, a good example is the word "black" to refer to people with dark skin; while in the US seems to have become a sort of taboo word, in the rest of the world, from China to France to Argentina to Russia and most everywhere else in between, the term is perfectly all right -a quick Google search will show you- and it does not carry any derogatory or demeaning meaning. Actually, it might do depending on the context (and/or tone) in which is used, but this is true with many other terms.
It's a good exercise to empathize with the culture you're visiting before making any judgement. In living memory, Spain hasn't had apartheid nor instituted segregation, therefore -albeit there's the odd loose imb***le - racial features have never been a problem in this society (again, we're talking in living memory, not in the 15th century... when it was!). In fact, much like in other big cities elsewhere, in Madrid (or in Barcelona for that matter) nobody cares much about your origin, your race or your religion providing you're a "nice" person. Thus, the importance you might give to a US poster from the 50s is far different from the importance a typical Spaniard would consider -moreover noting there are Spaniards from all colours, races and creeds. It's just a poster from the 50s, which might (or might not) have a purely artistic value. Period.
Obviously, Ling has the right to feel offended, and the choice to choose a different hotel... but, IMHO, she'd be doing it for the wrong reasons.
Ahh, and @vandrabrud introduced yet another controversial issue :).... how far has a host have to adapt to a guest...or vice versa: servility vs entitlement. Great topic, surely would bring passionate answers.