Mojang explicitly gives permission to mod minecraft on their terms page:
http://minecraft.net/termsAs far as I can remember, Notch had always been okay with it.
It's an interesting point about the ad.fly stuff, I don't know why the mod makers said yes when approached by the FTB guys to include their work when, yes, monetarily it meant the same thing...
To take a guess though, I would say it's a combination of probably not making that much off of the ad.fly links and out of hate towards tekkit's shameless thievery.
And you're completely right about the last thing... *fixes*
Tekkit Lite doesn't have most of my favorite things from FTB (FTB Mindcrack Pack, specifically):
1) Portal Guns
2) Twilight Forest (super fun when playing with others, raiding the huge towers)
3) Misc Peripherals (epic ComputerCraft addon, crosses over with a lot of other mods)
4) Omni Wrench and Forge Lexicon
5) Soul Shards (epic, but Maaaaybe a little Over Powered)
6) ExtrabiomesXL (autumn biome ftw!)
7) Forestry (Deliberately made incompatible with Tekkit when they would not remove it when asked to)
well... tbh, maybe forestry not so much... it's one of my favorite things to watch people get excited about on youtube- the bees part mostly- but that's way too involved for me to want to get into while I play.. You can do awesome stuff with it once you get going though.
Anyway, I totally see why people would stick with Tekkit if they haven't tried FTB and didn't care about the credit/fairness issues- I am subscribed to a couple people on youtube who play tekkit. It is fun to watch too.
:D