Google, Microsoft and AWS offer registrar services becuase it keeps you in their ecosystem for their higher margin products. THey generally offer competitive pricing for things like doamin registration and don't pull stunts like charging 2x as much for "privacy protection" or the even more dirty tricks like godaddy and other bottom feeders.

I recently trialed hosted email with AWS and while it is very basic it only costs 4/user/month - cheaper than my google apps service. I was also able to register a new domain at market rates and get dns automatically setup (I think?) on AWS as part of the service. Now because I tie my monthly AWS spend with my registrar I'm more confident I can get some customer service as well.

staying inside a vendor's ecosystem for very selective services can actually work out quite well, as long as the seller/customer incentives align and they are relatively commodity services.