This issue isn't unique to Poland, ubereats have been heavily criticised for doing it elsewhere.
Yes, it should be illegal and they should be prosecuted. No, it isn't currently illegal under the contract 'uber partners' sign and they won't be, since your contract with them wasn't a contract of employment.
However why were you all signing up for 5 day weeks if you were students ? Surely that is a serious breach of your visa conditions.
And yes, ubereats are happy to take advantage of your situation and doubtless dob you in to the immigration authorities should you attempt to sue them. There's a reason their reputation is so poor, a clue to this should be the fact that the CEO of Uber's twitter avatar is the cover of a book by the disgusting Ayn Rand.