Additionally, the union most poor people are likely to be forced into, if they are ever forced into a union, is the United Food and Commercial Workers, which is evil and will take $500 a year from you in return for "negotiating" a wage which is usually lower than competing non-union business. So given the choice between being forced to join a crappy union and keeping your pay, people choose to keep their pay.
Indeed. I was briefly a member of a union, and I just
loved paying unions dues to make $1/hr less than my private sector friend in a similar job.