1) You are violating the rights of the Mod developers IF they have said they do not want their works to be included in their mod pack. They don't really need a reason, its their own right so the least you could do is respect it. Your rights ends the moment the rights of someone else start.
2) They take away site/download traffic from the developers, this depends on what the particular dev wants, some doesn't have Donate links and they instead depends on ads (Like i told you, depends on the particular dev) or addfly links.
Quite honestly most of the arguments modpack (Or in this particular case technic and tekkit) fans is that they think they have more rights than the moddevs themselves and that it doesn't matter that modpacks takes the mod and add it without the devs knowing about it. You don't know if so much publicly actually hurts the devs or not, or that people rather download the modpack from somewhere else than the dev site/download links.
The devs are doing this for free, sometimes they ask for donations or put ads/addfly links so they can worry less about how they are going to pay some bills or buy food, for their own entertainment and learning, the act of coding can itself be fun and constantly coding its a good way to master or learn a lot more about the language (In this case Java), and the least everyone can do for them its to respect their rights and decisions on how they want things to go about their mods. Quite honestly is that something really hard to do?
And do note that the fact that its free doesn't mean its a free-for-all where you can do anything you want with it simply because its free, being free only means you don't have to pay any kind of money for it, but you are still required to follow any rules or guidelines that particular free things impose of you.