From the client side they "may" come back to it later or want to leave it open until they see how their current freelancer is doing. A good majority though won't hire anyone and won't close their job. Why does UW leave it open? If they don't, they would have to refund the the connects to the freelancer. By forcing the client to have to close posting, and knowing a very healthy number won't, UW doesn't have to refund those connects. That's it in a nutshell.
That goes a little far. You cannot just post anything and expect it to stick. There are rules and if the post violates them, they will be taken down. Otherwise, as long as the content isn't problematic, it will remain.