Pretty easy to figure this out.
The unions understand the difference between making money on volume and making it on margin.
They want to make their money (aka dues) on volume, so trying to create a scenario where employers stampede their workers into unions to avoid the higher minimum wage.
Good luck getting employees to join a uion that reduces their pay.
If I were an semi-worker who would otherwise qualify for a minimum wage, then what incentive would I have to even join a union under these conditions?