People don't choose to not understand an accent. It can be very difficult to understand a really heavy accent where words are pronounced totally differently.
How do you work out what the words are when they don't sound like the words you know?
My husband is Kenyan. When we first met, he had a very strong accent. There were many times I had to ask him to repeat something because I hadn't understood it the first time.
And then there was the Shepherds Bush Row 
where he argued that it was Shefferd because ph is ff. Quite a humdinger that was!
But you aren't talking about people who have genuine trouble and seek clarification - like most people do!
Your title and OP were very misleading and gave the impression that if you don't actually understand words said in a way you've never heard them said before - you're ignorant. Which is just dumb! There is no ignorance in hearing an unfamiliar sound and being unable to decode it and relate it to a familiar sound.
But you're not on about that. You're talking about prejudiced people who hear a foreign accent (foreign as in overseas or unfamiliar) and use that to be a twat. Normally a bigoted twat. And even though they can understand, they say spiteful things or sneer.
That's very different from actually not understanding what someone said to you.