The key question seems to be whether regulators look at DIRECTV and DISH as the only two companies in the DBS industry, or instead as two among many multi-channel TV service providers (i.e. including cable). In the later case, the combined company will still own less of the market than ATT.
Some rural areas don't have a cable presence, so the argument can be made that in those locales the merger reduces choice. On the other hand, since pricing will be on a national basis, those customers will not be victimized by localized monopolistic pricing. Put another way, pricing is determined by competition with cable on a nationwide basis, and the urban and rural consumers equally benefit from that competition.