Singling out the leader would definitely be unfair in a wrong way, imo. Leader and the one who withdraws - maybe, but not convinced leaving out the entire group of that would be fair either. But nevermind that, making it so would give way to incredible personal grieving, I think. Just not a good idea. Current system is fine as it is; perhaps not fair, but fairness is not a point of collective punishment, which is what this is called, a long-existing concept. Changing it in any way would be nothing but discrimination, with the sense of unfairness even more blatantly unnecessary for arbitrary reasons.
How would it be at all "discriminating" to punish the proper person (that is, the person/s that actually clicks withdraw or fails to click accept in 45 seconds) for their withdraws? This could have been easily made more fair, they just took the easy way out here because there likely was no coding in place for how to single out one person in a partial party and give only that person a strike, so they just made it apply to everyone.

When you have people hitting accept and still getting dinged with a penalty anyway potentially through no fault of their own, something is *wrong* with the current system.

...is there really so little content in this game that there is literally nothing to do for half an hour without the DF? At all? Really?
About the only worthwhile thing to do otherwise as far as character advancement goes is hunts, and either not everyone's into that / it no longer offers the sorts of rewards they want.