Meg Master's ghost in 4x02 blames Dean for everything that happened to her and for her death / not saving her and Dean accepts that blame because he's always accepting things are his fault and taking responsibility for when things go wrong, even if they aren't actually his fault. And Meg's death isn't his fault. And ghosts are not reliable, especially not the ghosts in this ep, who are purposely being risen to kill hunters.
Meg blames the Winchesters (but Dean specifically) for the fall that led to her death:
DEAN
I'm sorry.
MEG
Oh, yeah? So sorry you had me thrown off a building?
DEAN
Well, we thought --
What Dean "thought" was likely that the real Meg was already dead, since demon!Meg does not seem the type to keep her human host alive.
But anyway, regardless of if they thought she was already dead or not, Meg's fall wasn't their fault. They didn't "have [her] thrown off a building."
What actually happens is this:
SAM walks over to the altar and overturns it. Suddenly, the shadow demon appears and grabs MEG. She is dragged across the floor and crashes through the window, falling down to the street below. SAM grabs his knife and cuts DEAN free from his ropes. They walk over to the window and see MEG sprawled on the sidewalk, dead.
Dean isn't even an active player in this sequence of events. And even Sam doesn't do anything directly to Meg, he just destroys the alter. The Daevas then turn on Meg and they're the ones that throw her from the building.
SAM: So, I guess the Daevas didn’t like being bossed around.
Then, ghost!Meg in 4x02 tells Dean:
MEG
No, you didn't think! I kept waiting, praying! I was trapped in there screaming at you! "Just help me, please!" You're supposed to help people, Dean. Why didn't you help me?
DEAN
We didn't know.
MEG
No... You just attacked. Did you ever think there was a girl in here? No. You just charged in, slashing and burning. You think you're some kind of hero?
DEAN
No, I don't.
But in 1x22, Dean is the one who is most vocal about exorcising demon!Meg to put human Meg out of her misery once he learns that she's actually a human possessed by a demon (which yeah, until Bobby tells them that in this episode neither Dean nor Sam know). Anyways, Sam wants to keep questioning demon!Meg but Dean says:
DEAN: Sam, there’s an innocent girl trapped somewhere in there. We’ve go to help her.
Dean wants to exorcise demon!Meg but then Bobby stops them and tells them that the demon is likely the only thing keeping human!Meg alive.
BOBBY (comes up to them): You’re gonna kill her.
DEAN: What?
BOBBY: You said she fell from a building. That girl’s body is broken. The only thing keeping her alive is that demon inside. You exorcise it – that girl is going to die.
Still, Dean thinks keeping the demon inside this innocent human girl is unacceptable. Bobby counters that they can't kill her (the exorcism would kill human!Meg) because she's human and it wouldn't be right. But Dean argues that it would be putting her out of her misery (since if she really is so injured they wouldn't be able to save her anyway)
DEAN: Listen to me, both of you, we are not gonna leave her like that.
BOBBY: She is a human being.
DEAN: And we’re gonna put her out of her misery. Sam, finish it.
Then as soon as demon!Meg is exorcised and they see human!Meg is still alive Dean immediately tries to help her:
DEAN: She’s still alive. (to Bobby) Call 911. Get some water and blankets.
Even though it's unlikely she'll survive he still tries!
So, Meg's twisted ghost-perceptions are not fact and not even an honest version of what happened. Like most ghosts on the show, her perceptions have been warped and she's casting blame on Dean where it doesn't belong. But Dean's self-loathing just accepts this blame and accepts Meg's version of events because he feels guilty that he couldn't do more for her or save her in the end. But he did try. And he's not the reason she was injured so gravely to begin with. And really the only person to blame for what happened to her is demon!Meg.