That's exactly why it's done. The airlines effectively eradicate legal employment rights and hire at increasingly tougher T&C's. The crewing agencies take a share in the difference.
I'm very surprised that the US, with relatively liberal legal employment practices in general, hasn't yet seen operators go down this road. This is one of the foundations on which one of Europes biggest carriers, Ryanair, has built their operation.
Also, Norwegian Long Haul (NLH) are planning long haul operations with the 787 out of the US based on this model quite soon. I'm having trouble seeing how carriers with unionized pilot groups can compete with the cost structure of NLH with staff working for half the salary. That has been the paradigm in european aviation the last 10 - 15 years.