I don't know what Portuguese chain "Żabka"
It's not a Portuguese chain. Are you so long out of Poland that you've lost all concept of how it works? The name and underlying logistics is owned by a British company, however. It's a franchise network. The majority of them are owned and run by Poles, which explains why they're known for using dodgy contracts.
But as I said you have little knowledge about financial world plus you are liberal which means you are close-minded so you won't learn anything new that wouldn't suit you biased opinion.
Okay, so let's use some common sense, shall we? Tesco has a huge distribution network, and they buy in huge numbers. They're able to negotiate with suppliers to push prices down as much as possible, which translates into cheaper products on the shelves. Pawel's Sklep on the other hand has no bargaining power, so his prices are higher than in Tesco. But as Pawel's business is small, he can't afford to take the risk of employing people on umowa o pracę because of how much trouble they can cause him. He doesn't have deep pockets like Tesco to fight legal battles - so he uses umowa zlecenie.
Why do you want people to pay higher prices?
It's quite astonishing just how anti-free market all you PiS lovers are.