A-ads users' balances are automatically used to top up the depleting balances of their active campaigns (the ones with positive daily budget and activated ads).
In order to pause or stop the campaign, the advertiser has to set its daily budget to 0. It may leave a tiny unsused balance that is sometimes too small to get withdrawn via bitcoin transaction.
That is why we introduced a change: if advertiser requests a withdrawal from her campaign, the campaigns' balance will be returned to her user account (instead of bitcoin address). So even a single satoshi left on the campaign's balance can be withdrawn (and the campaign can be safely deleted after that).
This change doesn't affect anonymous campaigns that are not linked to any particular user.