They banned it.  And that has had major consequences for the liquidity that banks used to provide in global markets.  Hence, that is, in part, why the swings in markets have become so violent (post-financial crisis).  

Of course (related), it has also damaged their, historically, very profitable market making businesses (which is classified as "trading revenue"). 

The line of managing the risk of market making activities and speculative trading, by the big banks, is a blurry one.  And the Volcker Rule put the burden on the banks to prove that their trading activity is against their market making activity.  That weakened the market making businesses of the banks and increased compliance costs.  And the major Wall Street banks haven't been the same since.  If we look at the five largest investment banks, the total revenue from fixed income, commodities and currency trading at banks has been cut in half since 2009.  

Today, two of the five bank regulators approved revisions to the Volcker Rule that soften that scrutinty on bank market making activity. 

This is a big step in a process that could unlock tremendous pent-up value in bank stocks.