Okay, a challenge: Look through the fan card threads here and tell me there's not a single fan card that you feel you'd rather have had in the game than one specific official card. I doubt you'll be able to do that.
And it's not like I wasn't willing to freely give away any and all of my cards and many people here feel the same. Somehow, games like Smallworld have produced fan generated content en masse without any problem, so this legal reservation seems not at all convincing to me, especially if the ideas in question are offered for free.
I'll admit, having your Edicts officially implemented in the game would be really cool. But, in general, if you only get 1 expansion every year-ish (which as far as I know is by no means a guaranteed thing), do you really want it to be composed of stuff you could've already owned if you visited a local print shop?
Smallworld has different publishers who may be willing to take different levels of legal risk. Perhaps Dominion's various publishers aren't. Maybe if Donald X tried really hard then he could convince them, but is that really worth the extra work when he could just focus on making his own cards and not have to consult legal counsel? Also, we aren't qualified to speak to how many problems the Smallworld publishers/designers ran into along the way. If I were organizing a fan expansion for my game, you can bet there'd be an NDA in place, including a clause that says they cannot talk about the NDA.
Maybe you would be willing to sign a very broad agreement, but that doesn't mean it would be so easy to get everyone else to go for it. But all of this is kinda moot anyway, because it's not like he doesn't at least hear about concepts through the grapevine. I'm sure "An Action-Attack that gives out Debt" has either made it back to him via playtesters, or came into his head during development. The more likely assumption isn't that he isn't aware of the concept, it's that he tried it and didn't like it enough to include it over something else or is saving it for later.