Here's the most common reason why. Because the base station and handsets work as a set and changing one handset means that the new handset must be paired up with the base station the average consumer would only complain when they got the new handset and it didn't work because it was not paired to the base station.
It's really the best way to ensure the customer gets a working system.
Have other people had problems with panasonic warranties and lack of customer service? We purchased a panasonic phone system for our home. on one of the units the speakerphone button has stopped functioning (still under warranty) - they want the whole system shipped back including all manuals and cords. This will of course deprive us of any phone service as the other handsets work fine. When I asked the rep on the phone why we need to ship all units back his reply was - that's the way it is. This is a really dumb customer service policy. In fact on that basis I will never buy any Panasonic device again as it conveys a management arrogance toward the customer.