My *personal* opinion is that wise ISPs only punt customers to ARIN once they reach the point where they can, in fact, have a normal ARIN netblock assigned directly to them (currently a /20, unless I slept through another change...)
The guidelines have a strong preference for singly-homed networks to use IP address space allocated to them from their upstreams. I can think of no logical reason* an ISP would prefer their customers to go to ARIN rather than deal with them. The global routing table is better off for it as well, as the customer's /20 would be a new route, rather than being included in their provider's presumably larger block. On the other hand, I can think of many reasons a customer would prefer to deal with ARIN than their upstream, assuming the meager cost wasn't a factor and they don't mind polluting the global table a tad. Of course, that's not really an operational issue. DS * The only reason I could possibly think of is if the ISP is afraid that the large allocation will impact their future allocations because they don't have the confidence or competence to extract a proper justification from their customer and present/defend that justification to ARIN when their next allocation comes up. But this wasn't the reason you were thinking of, right?