Permissions may not be neccesary...tekkit seems to think that way...But mod developers invest a lot of their free time, resources and their sanity making something wonderful that they believe in and love in the hopes that people will think much the same they do when the final product is created.
Modders, in my opinion, have a right to put in any arbitrary restrictions on their mod they want. Don't like the restrictions? Don't play the mod. It's not like you need it to survive and many more mods have alternatives now than in 1.4.7.
At a minimum having people request permission to include a mod in a pack that will be widely distributed (aka public modpack) helps to bolster their desire to make their mod since they see people wanting it. At a maximum there are minor revenue streams from downloading mods (adfly being the most common) that pays very little, but at least it's something for their time and effort and modpacks bypass that and public packs expand that exponentially because the pack is open for the masses rather than just a small group on a private pack.
The revenue streams are in a huge gray area, and "seeing people wanting it" happens via other means too: OpenEye, IRC, forums, download analytics (from which I found out someone's adf.lying my mods, not good)... or even Googling. I do, in fact, google my mods once in a while to see where they end up.
Strict permissions ensure a few things from what I have seen one of which is quality (such is the case of XCW) making sure every tom, dick and harry aren't throwing their mod into anything that loads, another is, as keybounce has mentioned, to try to minimize the help requests because not every single person who can point and click is making a pack without knowing how to troubleshoot crashes before crying to the mod dev.
You have to make bad packs before you make good packs. Learning is a process and adding complex permissions on top of mods hinders that process, as people want to
play and not bother with our internal modder politics.
Besides, you're not obligated to be the one to help your users. In the case of my mods, many other people have submitted patches and even took up development of them in many areas; more support has been offered in #oc by random programmers for Computronics than in my own, official channel.
Personally, I have no problem with requesting permission from developers, my pack contains well in excess of 100 mods carefully chosen for a specific theme and purpose and I have taken great pains to work out any bugs, report those that are actually mod bugs or cross-compatibility bugs all before even releasing my pack to the masses to minimize those cries for help while also letting the devs of these mods know how greatly I appreciate the work they do and trying to reflect that in the effort i put into my pack. It seems a matter of common courtesy if you ask me.
I have no problem with requesting permission from developers either, but sometimes I find it easier to just pick a different mod and help its developer. That's why I like open-source mods so much: if I find a bug, I don't need to put extra burden on the mod developer; I can just go and fix it while he works on exciting new features.
Last words: I completely understand after having played minecraft and modded minecraft why so many good, hardworking devs leave, never to return, simply because of the vocal minority of players who feel self-entitled to use something they could never themselves EVER create on their own.
Now that is true. While I don't believe in complex permission restrictions as a good way of stopping it, I do believe that too many people in the community feel entitled to be getting work for what is essentially free. But we're not the only ones to have that problem and we need to stop acting like it: piracy is a
widespread issue which few people have been able to solve for themselves.