A discounted price (or rate) on a service is not the same as a "Special Promtional" item. The government negotiates discounted rates on everything they purchase. If you've ever worked in sales you'd know what the term "GSA contract" means.

What the rule quoted above refers to is the acceptance of special perks like free meals, gifts, etc. Accepting enrollment bonuses for a personal rewards account sounds like a gray area to me. I don't think it's fair because a venor (Marriott) shouldn't be able to offer a personal incentive like that to a government employee in order to encourage them to purchase their services.