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.