How about we forget about legal things? No modder is going to take a legal stand (they don't have the resources (or just don't want to waste them) nor the time/trouble it could cause). And no one is going to suddenly ditch Tekkit/Technic because it's illegal (those who would probably already have). Also the modding community does not exist of a single country, where I live just by downloading Technic you probably risk having your internet suspended (if the modder asks, which I doubt he/she would).
Its more of a moral standpoint. I'm a nice old lady giving you a free apple, in fact I'm going to give you as many apples as you want (I might make you spend 5 seconds of your time looking at something to earn me some income). My only request is that you don't give it to other people (you're free to send them to me, or to share it with your friends in private thou). Then another guy comes along, he starts taking all our apples (somehow bypassing the 5 second wait that earns me income) he then puts it with a lot of other apples (whoever said they had been forced to be compatible is wrong, its pretty easy to get them to play nicely) and giving them to other people. Without even asking, when some people request the removal of their apples from this "pack" they are refused. In the end one person has to put a chemical in their apple that makes it rot when put in the pack (the analogy breaks here, a rotting apple is far more harmful than what Sengir did) before the authors of the pack get their act together.
The users of this pack then come and flood the houses (which are deprived of users by said pack) of the original author asking for help with issues caused by said pack and insulting the few that remain thankful of the growers work.
Also Tekkit/Technic are hardly good packs. They don't launch properly which causes crashing, bugs and other issues. My friend downloaded it, and told me. I decided to just join him and play for a while. We crashed, glitched and things went haywire due to may issues in the pack (many of witch are very basic, beginner errors like launching under the wrong environment). We have since ditched Technic/Tekkit and are now happily playing on my own server (with the occasional bug, which I can quickly fix via configs or patches I can find online).