Should Brand Managers prevent consumers from sharing copyrighted images on social media? or should they encourage it for its marketing value?

As a creative person, it should be free of any charge and also sharing it to yours consumers makes your brand to reach more people which can be one of your future consumer. Just think of it your consumer as a free marketing tool. :)

It depends on the Management Cycle of the brand . If its in the first phase ( New Brand Building) , in order to Create "the brand awareness" , the social media sharings of the consumers can considerably help . But if its a globally known brand which is very well built , in this case a global brand can have very strict  marketing restrictions about social media sharings . 

Better not!  because, marketing is all about reaching consumers and enhancing the data base