One thing I've noticed is that people DO NOT want me to speak to them in Polish
Maybe they think you would be more comfortable speaking in your own language? Or maybe you are making more mistakes than you realise as you've admitted you sometimes mess up cases and grammar and they feel they are trying to be helpful ? I can't say I've noticed that Poles have a particular love for speaking in English. If I speak in Polish, then I'm answered in Polish. Of course, this is just my experience and clearly yours is different.
expats in other countries say how important it is to learn the local language; in Poland it is reversed
I'd say it is important as you won't want to be constantly reliant on other people, but I don't think it is reversed in Poland. I think many Poles are pleasantly surprised when foreigners speak to them in their own language. What you have to remember is that Polish is not an easy language to master, and very few foreigners probably speak it fluently, especially English speakers. Those from Slavic countries will find it easier to pick up. More than likely Poles are well used to their language being butchered ( I'm not saying that is the case with you though! ), and they know their language is difficult. I think more than anything that Poles don't expect foreigners to learn their language, especially given that English is becoming more widely spoken.