I think the reason is much simpler than that. Resetting the timestamp lets them easily resurface things on the frontpage, because the current time - posting time delta becomes a lot smaller, so it's again ranked higher. And avoiding adding a special case, lets the rest of the codebase work exactly like it was before, basically just need to add a "set submission time to now" function and you get the rest for free.

But, I'm just guessing here based on my own refactoring experience through the years, may be a completely different reason, or even by mistake? Who knows? :)