Getting too legally ,for me so I am out but I will just post this here. I cannot vouch for its validity, but the author seems to know what he is talking about:
"The mod authors are in the wrong in this section as they cannot claim copy right to their derivative work unless they take it to court and prove that there is enough separation from the main product (in this case, Minecraft, which is a hard sales since mods modify the gameplay of that software) to warrant their own copyright."
In addition, the modpacks use would still fall under the category of fair-use.