People born and raised in Mexico do this. It's because of the lousy plumbing, not because they like it.
And, I don't believe anyone here is really trying to blame mexicans, they're just spitting what they've seen.
I'm mexican raised here, and I never saw that shit until I went to see my family in Guadalajara. Then, when I first met my wife's family (who are from a small town in Michoacan), they do the exact same shit.
I agree, it's very bothersome when they do that shit here. I just let them know it's all good, but for some reason that habit is hard for them to break.