There's nothing wrong with selling mods and plugins, and, as said earlier in the thread, we already have places that already do the exact same thing. HytaleHub isn't the one selling plugins and mods, this section of the forums only exists so that developers have the opportunity to turn a profit on something they probably spent a lot of time on.
There is your TL;DR. Still don't get it? Think about it this way:
Imagine you've just created a super cool awesome mod for a game that is full to the brim with features, but has cost you a lot of money to develop and possibly to pay other people to create things like graphics for your mod. Some developers will release the mod for free anyway and accept donations, others will sell the mod, and some will sell the mod and accept donations. Free developers get nothing back from what they create except the love of the community and the donations, but developers who sell their work get back what they put in, as well as the love of the community.
Again, this is your super cool awesome mod that is totally awesome and completely free of plagiarism. Not all mods are like this. Sometimes, especially on sites like SpigotMC, you see some junk that someone developed that they are selling. If no one buys it, the developer is at a loss and cannot improve their mod. If it was free, people may actually download it and provide valuable feedback. It's a struggle to make the decision of whether to monetize your mod or to release it for free. Both ways have their own advantages and disadvantages, but in the end it's up to the developer who made the mod to do what he wants with the mod.
Note: I'm leaving mod as the generic term it is, a modification. I'm not worried about splitting it up into plugins and mods. They're both the same thing: a modification to the base game.