These partners use Delta SkyMiles as an incentive to get people's business. If these rental companies refuse to follow through on these promotions, then they are not only harming their own reputation, but they are also harming Delta's reputation.

Delta has clout which individual consumers do not. If one of their partners is not playing fair, then it is, indeed, their responsibility to take action -- either by putting pressure on them to post clear rules, award miles in a timely manner,etc. or by ending their partnership with them.