Calibre is licensed under the GPL v3, so they'd have to check with the particulars of the license. I have seen commercial software interface with GPL software before, albeit v2, so it is probably legitimate. The worse case scenario seems to involve open sourcing the plugin component itself, and I think that would only be an issue if they used GPLed code to make the plugin. (In other words, ensure there is a clear separation of the commercial and open sourced components.)