Because Apple ram is supported Through Apple and extends the whole three years if you pickup AppleCare or longer if you work out a service agreement.
For some of ya'll and myself included Third party ram is a good way to save some money since we know how to test ram for defects and call up the manufacturer for RMA work. Same for large corporations who have dedicated IT departments in every branch office; the savings are substantial for corps who want to do their own work and can often do it next day.
But small businesses and data center people will go OEM. Small businesses will be able to take things back to Apple and tell them to fix it. Likewise for Data Center folks, If you get the premium support for , say , the xserve, they will be out to your data center/colo in under four hours if something breaks down, and if its the memory they won't just tell you to take it up with the vendor you bought it from.
So if you are doing mission critical work then 100% oem is the way to go, unless you are tech savvy and don't mind doing your own repairs.