Jim,

Sorry if I'm about to make your head hurt

I don't think his position fits your slippery slope situation.

His analysis is a basic cause-effect approach to policy. It requires speculation, but all policy making requires some speculation.

The belief that a policy allowing non-attendee pre-registrants to receive their pre-registration package would result in an significant increase in future non-attendee pre-registrations is a reasonable one. Although it is possible this would not be the case in the scenario which you put forth were the Casino Freebies are removed.

It is not fallacy to reject a policy on the basis of projected negative consequences.

The slippery slope argument would be something more along the lines of:

Well if we do it for non-attendees who preregistered we have to do it for . . . and then we have to do it for . . . etc.