Disney takes great pains to ensure the cross-U folks are salaried cast members. That's the great thing about salary, you get paid the same- no matter how many hours you work or what you do. In short, they are not paid anything extra.
How does this 'free-labor' not rile the unions? Cross-U shifts only happen during peak times are used to help fill what would otherwise be 'open-shifts' when the park get's really shorthanded. At no-time do the cross-U shifts of the TDA'rs deny any unionized CM of their job or pay. Thus, the unions are perfectly okay with it.
"If we cut the budget are you going to be the one standing at the exit explaining to guests why the ride they just rode is a piece of crap?" - - John Lasseter