This is done so that whitebox system builders don't buy consumer level motherboards and CPUs, shove ECC in there, a cheap raid controller, and sell this setup as servers. Next thing you know all the AMD servers on the market are failing because of shoddy parts because consumer level QA is at a lower level than enterprise, not to mentioned the lowered performance of consumer level hardware. Now AMD has a bad reputation for reasons completely unrelated to its chip quality or performance and collapses in the market.
I don't like this situation, but I certainly understand it. AMD needs the server market to raise revenues. This means its server offerings, from soup to nuts, need to be rock solid. Letting whitebox resellers junk up their reputation is not within their interests.
Not to mention, enterprise sales often subsidize consumer sales. Those big enterprise markups go into the same pot of money that allows AMD to sell consumer chips at lower prices. Anything that threatens enterprise sales could affect the consumer space. AMD needs to be very careful here as, compared to intel, it needs to prove the value of its brand every chip generation. They also need to overcome a lot of bad mojo they've developed from questionable benchmarking and marketing of its previous two, or more, generations of chips. Bulldozer, I believe, had half the single core performance compared to the intel equivalant at the time, yet it benchmarked well due to its extra cores. The market doesn't forget shenanigans like these.
That said, the market allows 'workstation' level cpus and motherboards that support ECC. A little legwork can find you decent deals, but no, they're not going to be $59 NewEgg specials and you won't be able to use the consumer Ryzen on it. Consumer CPUs are almost always gimped anyway - smaller caches, disabled features, slower clocks, etc. The ECC limitation isn't too different from this, but it is annoying.