That drives me crazy too! Some shows are pretty faithful, but others are way off. The otherwise-wonderful Cuatro Estaciones en la Habana is especially bad -- it replaces all the local Cuban slang w/ standard Spanish, e.g., they'll subtitle imbécil when the dialog is comemierda. Why would you do that?!

Also strongly agree w/ GP; in my experience, you've got to hide the english to learn anything. Otherwise your brain isn't working in the target language.