The universe is just one enormous 'Soul Filtering machine'

[Succubus]

...Why wouldn't there be free will in heaven?...

Do we have free will to commit sin? In the unlikely event I find myself in heaven, sooner rather than later my eyes will wander and I'll be gazing at some woman's titties. Then what happens?

You there! Yes you! Succubus! I saw that!

Then god pulls a big lever and the trapdoor opens and I'm screaming all the way down. But wait, what about god's omniscience? He knew I'd do that long before he even created me, so there's no way he would allow me in heaven in the first place. It's all sooo confusing.
Now what about the rest of the sad bastards? According to the law of truly large numbers, all of them, no matter how saintly, will fuck up eventually and they'll get the big lever.
There's nobody in heaven, it's empty.