Because of a third channel that was recently deployed as stated in the articles and a bug in the firmware that causes them to reboot, which is why sales of the 6650 were halted. Apparently this bug affects unbranded e71-2 and the e71x.
Phones don't need to reboot to re-establish a connection to the network if it is lost. If it has to do that, then there is a problem with the phone's software. That's complete rubbish that they told you that as other Nokia phones don't have the problem in the same areas as the e71 (as stated earlier in this thread by someone who had an N97 and E71 in the same area).
If you are talking about an e71-2, which it sounds as the e71x was not available until May 2009, this is not an AT&T phone spec'd by AT&T and provided to by a vendor.
Whatever the situation is with the network, these phones do not need to be rebooting all the time to handle the network, that is rubbish. This is a problem with the software.
Hell, even the iphone had a problem where it would lose network connectivity due a bug in the software until it was rebooted, which had to be done manually. This is a software problem, and the latest iphone firmware lists that this problem is one of the fixes in that release.
But that isn't what's going on with the e71, it's just rebooting randomly in certain areas. Nokia needs to address this. It's as simple as that.