NCAA rules currently state that a DII or DIII school can move up to DI in one men's and one women's sport, but cannot offer scholarships in those sports (or a DI number of scholarships in the case of DII schools) unless they move their entire athletic department up to DI. There are two exceptions to this rule that apply to hockey:
This first was an agreement made about ten years ago that grandfathered in a number of programs that had always been playing DI hockey. Clarkson, St. Lawrence, RPI and Colorado College were covered by this exemption, allowing them to offer the full allotment of DI scholarships (Johns Hopkins lacrosse and Hartwick College soccer are a couple of other notable examples of grandfathered in programs). RIT and Union College in the ECAC, both DIII schools, were not covered by the exemption and as such can't offer scholarships. Union doesn't seem to care, but RIT has been campaigning to get there own exemption.
The second exception is an NCAA rule which allows schools to play up a division and offer the full allotment of scholarships in that division if there is no national championship offered in their own division. This applies to the DII schools playing in the AHC (and other conferences) since the DII National Championship was discontinued in 1999. Because there's no DII championship for hockey, those schools (Bentley, Mercyhurst, AIC etc.) are allowed to play DI hockey and offer the full allotment of DI scholarships (Atlantic Hockey limits the number of scholarships available for its members, but that's just a conference rule).