Hanlon's Razor, don't attribute to malice that which is adequately explained by stupidity (or in this case, just ignorance). There is no evidence to suggest that they are knowing or willing participants in this use of their vehicles, nor is there any way to verify that any random renter of their vehicles will not use their vehicle for illegal and/or terroristic purposes once in their possession. Short of black listing those renters when they get evidence it has happened and publically speaking out against it (which sounds like they have done), idk what else they could do about it.

Have you specifically been boycotting Ryder since the Oklahoma City bombing?

What would you like them to do? Seriously. What, in your mind, was the "right" thing for them to do?

Please remember that "not rent to them in the first place" is not an acceptable answer as they are not mind readers and would have had no way of knowing what the plan for their trucks were.