Because people play a game as they like. Just as a mod maker cannot sell their mod they are under no obligation to freely give it out. What harm would it do? They are not removing money from your pocket nor harming my life then do as thou wilt.
As I said, I never disputed that the mod makers are well within their rights to do what they do! I just see a marked difference in mod communities - for other games, you are not allowed to make money off your mods whatsoever, including paid link shorteners. In the Minecraft community, adfly-style extra income (even if it's just pennies) is just fine - hence my curiosity whether that might have contributed to that difference; perhaps it's just purely coincidental and Minecraft just attracts a different type of players and modders, I don't know!
And note, I'm not just talking about the older Civ community, look at the current Skyrim community! Whether on Steam or the Nexus, people don't seem to have huge issues with collections and recommended mod lists (assisted by the fact, of course, that it doesn't require re-hosting the mod, but just a checkmark on a downloader) like we see repeatedly in the Minecraft community. *shrug*
EDIT: Also, lately, I've noticed a bit of a change in the Minecraft community itself - mod makers seem to be much more willing to give permission than they used to. I think FtB really helped there - and I think the Technic platform could help to make the community a more friendly one, too (if people are willing to forgive the initial controversy and hiccups of the Technic pack).