If they don't then the pilot has a regulatory obligation to challenge
the lack of an
altitude assignment

Unless they've changed 91.175, it merely says that if an altitude isn't
assigned when an approach clearance is received, the pilot is to
maintain the last altitude assigned. Where is the regulatory
requirement to challenge the lack of assignment?

When this change to the AIM and ATC Order was discussed, I think
everyone envisioned 2,800 being assigned for a direct-to DOCAP. But,
once the real-world takes over...

I'm not clear on how this clearance relates to the new change to ATC
procedures. That pertains to direct to IF's, but this fix is a
combined IAF/IF and has been for years, most likely prior to the AIM
change, and a clearance direct to an IAF has long (forever?) been ok.