I expect those whose native language is English to have an excellent grasp of it. I don't have a problem with non-native speakers of the language mangling it.

It's all part of the learning process. And chances are that no matter how long you speak it, you'll still have idiosyncrasies to your speech patterns because you're not a native. There's no point in criticising someone for that.
It's very difficult to gain confidence and ability in speaking a language if people are constantly criticising and mocking you for it, so I wish people didn't do it. I'm trying to learn Norwegian at the moment, and got teased about my poor grammar a few days ago, which really stung. I'm really, really trying, but with four other languages in my head, things are going to get mixed up for a while still. So I empathise with your situation.
