The National Council on Teacher Quality reacts to the NEA opposing TFA. The NEA claimed TFA is often used to reduce teacher costs, but the NCTQ responds that if TFA teachers are paid standard wages and benefits, savings would be marginal. (As an aside, many unions are outraged that districts pay TFA for each teacher placed, claiming that's wasted money. Well, which is it? Is TFA bad because their teachers are more expensive or bad because their teachers are less expensive? You can't argue both.)
Other NCTQ musings:
The number of TFA teachers across the country is so small that their impact is unlikely to alter unions.
95% of corps members' principals viewed them as effective as other beginning teachers coming from traditional certification programs.
While the NEA claims programs like TFA lower the standards for entry into teaching, it's clear that's just not true.
In my experience, many teachers are misinformed about Teach For America. In my second year at my placement school, I had teachers who thought that Teach For America paid for my housing expenses, that the school district didn't pay me, and that all Teach For America teachers were told not to join the local union. All untrue (though I wish the first one were reality!). In fact, the majority of my Teach For America friends who taught in my school district were union members. In our schools we were regular teachers. The divide between traditionally trained teachers and teachers who come through alternative programs (TFA, various teaching fellows programs) isn't there. All answer to the same people and work for the same cause.