I would imagine it's easier for the local bike shop to replace a complete motor under warranty, if it's under warranty it's getting replaced anyway. Not all warranty replacements will be for the bearings. Bosch then get the faulty one back and THEY are the only ones who really know what the faults are, this way they can tell if there is a design fault or they need to modify the build process etc. This stops bike shops finding "common" problems with motors etc and "passing" this info to customers inadvertently. I am assuming.
This is quite interesting.
If my bearings went and it was out of warranty, I would certainly strip the motor and try to replace the bearings, whats the worst that can happen... you have to buy a new motor anyway.
I find all those items which say "no user serviceable parts inside" are quite easy to take apart and fix. I work on the principle of ...if it can be built it can be dismantled and fixed..
or at least I will have a dam good try. Assuming you have the tools and some mechanical knowledge. If you are not confident, then by all means ask your local bike shop.
It's just a way for manufacturers to keep a lid on any potential design problems, keeps it all "in house"
Manufacturers of all types, cars, bikes, washing machines etc are keen to keep repairs "in house" as this is a very lucrative market for them, selling replacement parts etc, they can make more money from that, than from new purchases IMHO.