Frontier has the frequest flyer base and can afford to match prices MKE-XXX-LAX and most west coast destinations via DEN/MCI. Granted Airtran and Southwest will probably do fine on those routes too but the point is a non stop MKE-LAX is less important to maintain in the slow season if the route is only marginally profitable and pick it back up when the demand is there. It would be different if they did not have the connection opportunites (like Airtran) because you wouldn't want to drop the LAX destination completely.