Oh I don't doubt you - it's just that it doesn't make any sense. Maybe there's an angle here that I'm missing.
The only possible benefit to the unions is that the illegals will most certainly vote for Democrats and are very pro-socialist. That helps them in some ways, but the illegals will also bring down wages which hurts them even more.
Since we are in a global economy, even turning the country socialist doesn't help union workers as their products would be unable to competitively priced.
All union members pay dues.
That's the bottom.
Union memberships also get political attention as voting blocks.
Give the illegals union membership and the right to vote and the diminishing union membership increases in strength again.
And so does the income of the unions.