since we live in a democratic society...
a) Poles, whether they behave as such or not, are mostly Catholic. That's the biggest group. The biggest group in a democracy gets what it wants.
b) In a democracy you're free to choose who you want your lawmakers to be. People here keep voting for Catholics; in fact, for some strange reason, it seems you NEED to be Catholic to get the job.
So as far as democracy is concerned, everything's fine. But, if SeanBM is right and there's no law* forcing them to close, the financial democracy known as capitalism means that the first shop that opens on these days will profit, and all the others will either have to change their ways, or go out of business. It's just that no-one wants to be first.
As for planning your shopping: I work during the week, often into the evening. Local shops don't even open until 10am on weekdays, and that's just what the sign says; I've had to stand waiting outside long past 10am until the slovenly wench who's supposed to serve me pet food and muesli bothers to unlock the front door. Poles won't learn how silly this attitude is until the big 24-hour supermarkets have forced small shops out of business, like they're doing in England, and then it'll be too late. I like my local store with its odd selection of cakes and unpasteurised beer, but if they can't be bothered to pay a student 6zl an hour to man the tills in the morning, or evening, or Sundays, I really can't have any sympathy for them.
*the actual law is about National holidays, not Sundays, right?