Why would governments magically stop subsidizing Amazon if their workers unionized?

There are plenty of reasons to support unions, but this sort of corporate welfare could be eliminated by increasing the minimum wage.

Paying people minimum wage is "corporate welfare"? How so?

Because the alternative is the taxpayer winds up paying for those workers’ healthcare, food and housing (Medicaid, SNAP and Section 8 vouchers).

You're forgetting the alternative where the person ends up getting unemployed instead, in which case the taxpayer pays 100% for those workers’ healthcare, food and housing.

It isn't a binary, the question is the right representation as much as anything else.