And they know that their Polish students won't realize the quality (and lack their of) of teaching they received until it is too late.
I say, that seems a bit mean!
Ok, you have a point that there are numerous Brits 'teaching' English abroad who do seem to nurse almost daily hangovers, and who have dropped out in one way or another ... but I think nowadays they do actually need some kind of teaching qualification, don't they? Or at least have been on the 4-week TEFL course. Well, any reputable school would require that.
I taught English in a very famous UK school in Poland back in the 1990s, and yes, I saw a very motley crew of Brits in the ranks, particularly at other schools that weren't so fussy in their recruitment. And they all lived in the bars, and spoke pidgin Polish and were a bit embarrassing, really. I went native and pretended I didn't speak English ...
HAVING SAID THAT, I also came across A LOT of Polish English teachers who couldn't actually speak much English. Seriously. Not just the accent - all of the trimmings such as articles, correct tense usage, and other such niceties. They were prepared for lessons and didn't stink of last night's vodka, but the English they were teaching was suspect, to say the least.
But that was ages ago ... I though it must have all changed by now?
One important point, though: nobody can hope to teach a foreign language out of that country (e.g., teach English in Poland) and for students to perfect that language. It is impossible. You have to live in the country of origin to have even a chance of getting to the higher levels of real fluency. So: the aim of learning English in Poland has to be competent communication (someone knows you're not English, but still understands you, and you them). In which case, if those Polish English teachers were just a bit better at English, they would probably best best for the job ... (and I'm sure thousands are, nowadays, 99% perfect at English ...).