Uh no; they can't. Or more precisely: It probably very much depends on the jurisdiction (or country).
The absolute fundamental basis of contract law (at least around here, but I strongly suspect this to be a more or less universal concept) is that a contract is negotiated in good faith from both parties.
I'd wager (and no IANAL) that a bait and switch scheme (and it's exactly that when a contract is suddenly unilaterally changed on the whim of one party) is fundamentally contradictory to this very concept and thus invalid.
Sure: They try it anyway, but I would be very, very surprised if such a scam is upheld by a court.
The absolute fundamental basis of contract law (at least around here, but I strongly suspect this to be a more or less universal concept) is that a contract is negotiated in good faith from both parties.
I'd wager (and no IANAL) that a bait and switch scheme (and it's exactly that when a contract is suddenly unilaterally changed on the whim of one party) is fundamentally contradictory to this very concept and thus invalid.
Sure: They try it anyway, but I would be very, very surprised if such a scam is upheld by a court.