I've heard people air complaints about this sort of thing before, and my reaction is usually some mixture of amused indifference and a feeling of "I don't care enough to argue with you about this". Frankly, I think it's a little silly. Yes, it can be very frustrating (not to mention insulting) to be treated like an idiot, and some people are very guilty of assuming foreigners are of below-average intelligence, but there's an important fact that I think the other noders here are forgetting:

The classroom is not real life.

Think about that for a second. Unless you've been studying a foreign language for a long time, the language you hear in the classroom most likely is NOT fully authentic. Since the most common method of teaching a language is to just explain to students how it works and let them practice, teachers tend to avoid explaining too much at once. The result is simplified grammar and a disregard towards subtle nuances of speech. More importantly, the speech that students hear is often pronounced slowly for the benefit of the students.

People talk at different rates. Two people engaged in a long-winded philosophical debate will usually speak slower and enunciate more than two people having a heated argument over a very emotional topic. The philosophers will probably be speaking more softly as well, because there are strong lines of communication already established. Conversely, people will often raise their voice when they feel they aren't being understood, whether they're speaking to a foreigner or a family member.

When learning a foreign language, there are a lot of things going on behind the scenes. Not only does the student have to get a grip on the language's grammar and vocabulary, they have to become familiar with the language's phonetic inventory (ie, what sounds the language uses), phonotactics (what combinations of sounds, in what order, sound natural to a native speaker), and even where one word ends and another begins (there's a reason people who have never studied the language can't do this). Add to that the fact that a student of a foreign language is probably more used to the slow, enunciated speech of the classroom (at least early in their trip), and it's somewhat understandable that people slow down their speech for foreigners. Anyone who's ever studied a foreign language and struggled with listening comprehension homework has probably realized, at least subconsciously, that slowing things down can be helpful.

Now, should a native speaker get pissed and refuse to explain what a mop is? Of course they shouldn't. Just don't yell at me for being an ethnocentric jerk when all I'm doing is trying to make sure I'm not speaking too fast.