The store didn't voluntarily agree but they were forced by the NLRA to accept a "good faith contract" negotiation that the store had no interest in having good faith with. This violence-enforced negotiation process did result in it becoming a union represented shop. So the union via the NLRA forced the workers to pay dues to the UFCW (technically you can decide not to join while still having to do all the things union workers do including paying the dues).

So yes the false premise here was it was just a voluntary contract, but it really wasn't since the law required them to act in good faith to come up with a contract with the union when they had no interest in doing so except for the violence of the state that forced them to. Otherwise they'd have just hired non-union workers.

I did explicitly ask them when I was hired that I absolutely did not want to join the union or pay the dues. The manager was happy to hear that because they didn't like the union either, he certainly would have hired me without requiring anything union related if not for the NLRA forcing the "good faith" negotiation blocking it.