How would the union know what the boss tells his new hires? Unions don't get to monitor the hiring process of every communication between a worker and his employee.
As for how they'd know it was going on: it only takes one person to "leak" that information to the union. And if the union and company are large enough that new people get hired regularly, they might occasionally find someone who fits the profile and send them through the hiring process specifically to find out how a new employee gets treated.
Any argument you make will always come back to, "The company will threaten and fire people who don't go along with what they want."
The whole point of a union is that employees have more power collectively than individually. A company can just tell an individual "no". If all or almost all employees get together and say "the situation is so bad that we're willing to quit (and make a lot of noise and bad PR for you) rather than let this go on", then that's harder to ignore. If the company is willing to fire everyone and deal with the bad PR, and that's cheaper than the union's demands, then the union doesn't actually have the power to effect change even as a collective.
I don't see any reason that either the company or the union should be artificially given more power in that scenario.