Pretty much this. If they don't protect it and can't plausibly claim to not know about the breach, that's basically an admission that their trade-dress has become generic (kinda like band-aid or jello). In other words, they have to move to protect their trademark, even if they don't actually care about the breach itself (for example, the fact that Nintendo waited as long as they did before moving to shut down AM2R kinda implies that some Nintendo exec liked the project and delayed the legal response for as long as possible).
I honestly suspect that this is the real reason why WotC employees aren't supposed to look at custom magic forums — if they hung around somewhere like this thread and saw our sick CC renders and heard people talk about how you could print them off on MPC, there would be legal issues if they didn't report that to their superiors. And that would hurt public good-will towards WotC, so it's better to just make it a matter of policy that you can't — that way, you can act shocked (shocked, I tell you!) when someone tells you about the stuff you already know about (but can legally claim that you didn't).
It's not like copyright, where it's up to the copyright holder to decide whether or not to go after you (and generally doesn't unless you're seriously pissing them off).
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And, I mean... it also directly violates their fan content policy. Which... hey, it looks like CardConjuror literally just needs a written note from WotC saying that it's OK for them to operate. It shouldn't be that hard for them to get their hands on one, given the community backlash against mean ol' WotC.
(Fun fact: the fact that we have mana symbols as emoji technically violates the fan content policy, and WotC would be within their rights to shut us down for it. So no-one tell WotC!
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