That question is posed in this Reason item:

Despite being in good financial standing, adult film performers and others in the porn industry have had bank accounts abruptly terminated—and the U.S. Department of Justice (DOJ) may have had something to do with it.

Under “Operation Choke Point,” the DOJ and its allies are going after legal but subjectively undesirable business ventures by pressuring banks to terminate their bank accounts or refuse their business. The very premise is clearly chilling—the DOJ is coercing private businesses in an attempt to centrally engineer the American marketplace based on it’s own politically biased moral judgements. Targeted business categories so far have included payday lenders, ammunition sales, dating services, purveyors of drug paraphernalia, and online gambling sites. …

For years, various government initiatives have been aimed at reaching the “unbanked” and “underbanked.” Federal officials claim to want to help these individuals avoid high fees and other downsides of nontraditional financial services, but it’s hard not to suspect these efforts have at least as much to do with wanting a record of everyone’s financial goings-on. If the unbanked were such a real concern, why would federal agencies be simultaneously encouraging banks to drop more customers? …

In a March 2013 hearing before a Senate Banking subcommittee, Sen. David Vitter (R-La.) pointed out the obvious: that DOJ has “no statutory authority” to be doing this. But why bother with statutory authority when you can just secretly strong-arm highly regulated businesses into doing what you want?