"Secondhand game sales were not a factor in this development decision," said the company, "so we hope that all our consumers will be able to enjoy the entirety of the survival-action experiences that the game does offer."
If used games aren't the issue, what is? Is there a technical reason that's preventing them from including data reset? That part's not totally clear, but here's what Capcom said.
"In Resident Evil: The Mercenaries 3D, all mission progress is saved directly to the
Nintendo 3DS cartridge, where it cannot be reset," said the company. "The nature of the game invites high levels of replayability in order to improve mission scores. In addition, this feature does not remove any content available for users."