No they actually can't because all those devices aren't even running the latest version of 6. New certificates were deployed for all devices running the latest version of 6 which is 6.1.6. This means that all the iPod Touch 4th gen's out there are using FaceTime without a problem. Everyone else on an iPhone 4, iPad 2, or 5th gen Touch or higher need to upgrade to the latest available version, in this case 7.1.1 to renew their device certificate.
What you are asking is for Apple to update an earlier version of 6 (6.1.5 or earlier) that is not even the latest iteration of that version (6.1.6). Seems unreasonable to me for any company to have to or be expected to expend their resources on forking older versions of old software when those same resources could be utilized for innovation instead...