I've be looking very closely at the terms of my warranty to see if they even have the right to deny you. I doubt they do. If it dies and they try to deny you, I'd be demanding a refund, under threat of legal intervention if necessary.
I'm with everyone else. Only four points of intrusion. Radiator (now bypassed), dipstick tube, somehow an improperly installed vent tube, or AAMCO is lying to you.
I've start with the dipstick tube. Take some kind of plastic bag and zip tie over the top. That will stop water coming in the top of the tube.
Vent tube is a little harder to check. I'd request they reroute it while the trans is out this time. I have mine run up under the intake manifold.
And AAMCO, I've be checking my fluid a couple times a week to see if it's starting to turn strawberry milkshake again. Also, like has already been stated, make sure they are flushing the cooler lines *and* replacing the torque converter every time.
*If* it fails again after all this, I'd be asking them to use a different core. Offer to pay their core charge, but do not pay for another rebuild. If there's a problem with the core they should have found it while it was apart. Whatever the problem is they should have found it by now.
All else fails, get your refund and head to Amarillo. Those Precision boys are pretty slick.