I'm not going to bother with a toy analogy. Once upon a time, WotC descided to take the prudent step of creating a "safe harbor" for third party publishers. This practice aknowledged their obligation to allow others to create compatible and competitive products. Just like Hoover and Volkswagen and other companies, they realized that restricting that activity was not only impossible but ultimately unethical. Then one day they said, "It's my preciousss!!!" and attempted to drown third party publishers, the same guys who had supported, promoted and help developed their products, and in many cases even worked for them.
The only reason WotC doesn't risk being sued/fined into oblivion over this form of IP racketeering is that the gaming market is too small for that to be realistic.
All WotC really owns is the specific text in their books, and their individually developed trademarks. One thing they do not own is the "System." A game system cannot be copyrighted, and only its true innovations can be patented.