Be careful with bit.ac. I heard that their users cant withdraw their coins from the site and their support is not answering any emails. Im not saying they are but that might a start of an exit scam.
Where do you get the information about the user's bit.ac can not do withdrawal of bitcoins? You must give us more information about your claim, it will makes everything clear for user's bit.ac because what's i know about bit.ac everything is fine never find the problem and i never find the complaint about issue on withdrawal on there. Thank you.
No solid evidences from me as I dont use the site regularly. I used them once when I was in their signature campaign but thats it. But there are several threads that were started by bit.ac users. I suggest you use the search box and see for yourself.
Are you a regular bit.ac user? How can you tell that everything is fine?