It's a shaming tactic, more for the retention of remaining members than as a direct attack on the apostate.
Six months after leaving my last church and coming out as an atheist I ran in to a "friend" from church, who stood and stared at me gob-smacked, and eventually explained he was amazed how well I looked, having left the church he expected me to be unkempt, unhappy, divorced, corrupted, drunken, depraved and sin ridden.
Yet there I was in work, successful, very happy with a new son and still a non drinker/smoker, I could see the "does not computer" steam coming out of his ears and the fight going on internally as to whether to run away screaming or stay and try and find out why I was not a "literally" godforsaken mess.

Let’em assume whatever they want to.

Enjoy your sinnin’.

Don't start without me

Where is all this sinning going on that I missed out on? I must have missed that memo from Atheist Central!

You ARE accountable, but not to 'god,' in my opinion, but to 'natural law.'

"Sin requires the existence of a god. And your falsifiable evidence to support your existence claim of your particular god is what exactly?" soon gets them riled.

These sort of posts are usually greeted here with a tsunami of modesty, (often fake PC modesty ). But I ask you to think on this and tell it to your religious friends if needed.

Most people are born into a religion, therefore most sceptics have left a religion, and the reason for doing so is, usually, lack of evidence. But the 'motivation' for taking such a large and often difficult step, is most often moral feelings. Therefore there is a clear case for claiming that in most cases sceptics have finer and stronger moral instincts, because of the clear action they have taken.

I didn't become an atheist just because I wanted to sin and not be accountable to a god. I also wanted to sleep in on Sundays.

I'm tired of the assumption that belief (or non-belief) is a choice in the first place. I mean, one is convinced by evidence, right? I guess calling it a choice is intended to shift the responsibility, but it's dishonest.

Can’t be accountable to what doesn’t exist. Sin isn’t a thing either, so actually, we are accountable to OURSELVES.

I hear this statement as well as the assumption that deep down I still believe in a god.