SUPER IMPORTANT CLARIFICATION/UPDATE: In a statement we have received from WhiteDove herself, she has asked us to clarify that Hasbro has only contacted DeviantART and asked them to ask her to remove her commission prices. Nothing else has occurred, heavily implying Hasbro is content to pursue the barest minimum of legal actions for all the reasons listed below. A Lyra-approved thumbs up to them.
Earlier this week, Equestria Daily reported on Hasbro's Request of takedown against WhiteDove's DeviantART plushie commissions. The article erroneously cited trademark law as an explanation for Hasbro's actions.
Trademark law does require vigorous defense in order to maintain the holding: it's "use it or lose it", and very very harsh. But it would only apply in this instance if WhiteDove were to place official MLP logos on her work, or were to otherwise claim she was selling official merchandise. This is not the case. Therefore, copyright law applies instead. A copyright is issued at the creation of an intellectual property and granted to the legal entity with ownership of that creation without them having to do anything. It does not need to be defended in order to be maintained.
However, failure to defend your copyright voids an important protection in the event of a large-scale infractions. Namely, any lawsuit resulting from such an event would be limited in scope to the damages incurred by the copyright infringement. If Hasbro continues to actively defend their copyright, then they are entitled to seek the value of a license the franchise (in any and all endeavors), instead - an estimated cost difference of $100,000,000. The true figure may be more or less, but it gives you an idea of how much less effective a deterrent such a lawsuit would be to potential rivals.
Regardless of the motivations behind the cease and desist, WhiteDove would be able to retain her ability to sell plushies if she acquires a license to market products from Hasbro. Since the initial article ran, multiple helpful bronies have suggested this to her on her DeviantART profile, and she has declared in the comments section her intention to seek such a license. We would like to wish her good luck.
A special thank you to superpony and legal expert Anarion for contacting me with clarifications and helping to ensure Equestria Daily does its part not to spread misinformation.
Edit/Addendum: It turns out that Hasbro actually claims Trademark over each and every individual character. This means that Hasbro is motivated on both sides of the spectrum to act in many individual cases, as failure to do so would not only cost them millions, but potentially lose their ownership over Twilight Sparkle or her friends. The long and short of it is that this situation is massively, massively complicated, and anyone who commissions FiM works or is considering it should take this information into their calculations.
Note: White Dove did not recieve a Cease and Desist directly from Hasbro. Deviant Art asked her to remove the plushies after being contacted themselves.