TNA got what they wanted; a good deal of their talents were locked into either going international, doing indies, staying with TNA, or not wrestling at all. Devon, Matt Morgan, Sky, all came back. Shelley chose the Japan route. All the while TNA was free to pick and choose WWE talent that comes over, while WWE treaded lightly.
In addition to what PSV said, WWE could just refuse to sign any TNA talent, and instead, just go after indy guys(Sami, Del Sol, Generico, Briscoes, Cole, Reyes), leaving TNA guys to stick with TNA(Probably with some animosity), or try to get the guys WWE has yet to go after.