What about that justifies calling them "shady"?

Clearly Piko are paying the original owners for the rights to the games. People who want to preserve games could have similarly formed a company to buy the rights to publish the originals, but from what's being said about C+D notices it sounds like they didn't. The tweets from StoodUpOtomo go further, complaining that Piko might use the funds from selling this to buy more games in future - this implies that there are games which the preservationists could still buy publishing rights for, but that they won't, and they're attacking Piko to stop Piko paying the owners.