What someone does in their day to day life is totally different than what they do at work. It's no one's business, whether the government or a fellow resident (even if that resident happens to be a citizen), what language or dialect someone uses. It would be something else entirely if people were expecting British people to speak their language or learn their dialect. We aren't talking about Americans insisting that British people start spelling it "tire".
Not everyone who comes here to live does so to become British-like. It's long been questioned whether a multi-cultural society can expect their immigrants to assimilate or if the culture should instead change because of immigration (think a bit like the melting pot). A country which has immigration is going to change, even if most are mostly assimilating. It's up to the UK (and other places) to decide whether those changes are worth the benefits of immigration or if the drawbacks of pulling up the drawbridges and stocking the moat with alligators would be worth it.
I personally love British culture, and I don't think they are wrong when they say things like "left ten nant". I just hope that when I accidentally say "loo ten ent", and they know what I mean, a big song and dance isn't made of it. If you're my boss, sure correct me. If you're some woman ahead of me in the queue at Sainsbury's, please don't get all pedantic on me when you overhear me.
We're not exactly talking about people advocating dangerous activities here. We're talking about a language that for the most part is understood most of the time on both sides of the pond.