Teachers correct students because, well, it's our job. The truth is that we correct you students for a lot of reasons. We have to do it because you need to know when you're doing something which could make you look stupid or prevent people from understanding you.
We don't correct you to humiliate you. We don't correct you to make you feel bad about your English; if we keep correcting you about a mistake you continue to make in class, that's a sign that you need to start paying attention to that word or structure and change what you're doing.
We don't correct you to start a fight. Of course, if you want a fight, I'll give you one:
"That's not how you say that in English."
"But we're not in England!" And you get a smug look on your face because you think you've made yourself look smart at my expense.
And that's when I say something that's really bad in Spanish and ask you, Can I say that? Do you understand what I mean when I say that? Because I know that I can't say that. What do you gain by doing something that you know is wrong?
You pay me to be honest with you. You pay me to give you quality service that is meant to help you improve. Not telling you when you make a mistake isn't just bad service: it's dishonest. It's counterproductive. It's unethical.
I understand that being corrected can be frustrating and, at times, embarrassing. That's why I don't correct every single thing you do wrong. But please, if there's something wrong, pay attention to what we're saying. If you choose to ignore what I tell you, one of two things is going to happen: either you're going to get bored of hearing the same corrections over and over again, and you'll quit; or you'll just stop talking, rather than using the tools you get in class.
It's your choice. I can't choose not to correct you, though. That's not my job.