They get paid, they just don't get paid the "full" amount unless it's more than one accounting month. 29 day deployments are always the joke because then the Army can dodge paying BAH (housing allowance) and full deployment wages. Reactivating them on day 31 is diabolical though.

Usually they rotate a different units instead of doing consecutive domestic deployments with the same unit.

Okay, that makes more sense, although still sounds stingy. Australian army reserve benefits kick in from day one (& is tax free), but the levels of bonus for field pay only starts after a few days (as in, only a few days, substantially less than 30).