They are free to be. When joining the EU, Poland accepted to join the monetary union at some point, but there is no date set, so in theory it can be infinite. And with its current strategy to drastically modernize and increase its military force, there is no doubt it will not met the financial criteria anytime soon so nobody is worried about that.
That's an interesting perspective. From what I see in the news, the EU isn't as happy with the outcome.