That is most unfair legacy. How it's possible the original core software to allow modding and 3rd party mods to forbidden? Is anyone familiar with legal aspect? In my opinion, any additional mods are inherit same rights like core software. I have payed money for core software not for mods. Mods are free and has no contract behind them.
No offence, but union of people moders is cool, mafia is not.
This is a decision made by each modder. FC is not even accepting donations, Alblaka does not use use Adfly, but accepts donations and some people use adfly. Actually a modder could even charge you for their mod, if they don't use base-classes, which most modders don't do, thx do forge and modloader.
So actually it is some kind of generosity that modders don't charge you. Eloraam could do it and Alblaka could do it too. Mohjang might take steps against it (it could easily shut down forge, which would mean, that the mods won't work), but it is not something that could be forbidden.
Like Mohjang decides about what you are able to do with their game every modder decides what you are able to do with his mods. And while Alblaka again has a rather openminded attidude towards this there are also modders that are more restrictive. Some post restrictive rules, because this would make everyone need to talk with them, which gives them more control.
While you are believing there is no contract behind the usage of a mod their actually is one. If you download a mod you actually agree to accept the terms of the modder (this has it's limits of course, but by downloading you are accepting their copyright). Technically a mod like IC2 or RP2 is not a 3rd-party-programm. It is individual code, which actually does not even depend on minecraft directly, but on forge. From the legal view this is a bit in the grey area since these mods don't modify Mohjangs code, so they don't fall under their copyright.
So every mod created is individual intellectual property owned by the person or the group of persons creating it. Even if you create something you are not allowed to create it is your intellectual property, which actually is protected by law. And you are allowed to use it for yourself. So if you decompile Redpower to create a mod for yourself you are allowed to use it, but not to share it (basically if you ask eloraam to make some small adjustments for your server she normally let's you do it, or even does it in her files, if it is reasonable).
So to sum it up:
The basic rule is that everyone is the king of his own work. This is true for Mohjang and Minecraft as well as for modders and their work. No modder is forced to mirror Mohjangs copyright law or anything else, they are free to decide for themself.