Why most americans think every non english speaker speak english with a mexican accent?

http://www.youtube.com/watch?v=TS4v_kj9rw4

the inability to pronounce the R's (the easiest consonant to pronounce) and ae's. always that same peculiar rythm.

why is it? 50 million of mexicans in a country with 300 million people, that's why they think that everybody is mexican?
if u can pronounce the r's and ae's properly NOBODY will think u are mexican.

but in american movies fe, every non english speaker speaking english speak like the mexicans.

How do you ask someone to speak more slowly in Spanish?

I'm an English speaker learning Spanish. When I meet a Spanish-speaking stranger in public, they speak faster than I can take in the words. How can I politely let them know I'm a rank beginner and not very fluent yet?