[Tristan Lall]

If I had to guess, I'd say it's an oversight rather than a deliberate safety-oriented ruling (presuming that tubing was considered a fitting last year—I don't remember the specifics). But the ruling is nevertheless clearly stated and enforceable.

Accidentally deadheading the compressor isn't such a terrible thing. Certainly the compressor will stall, but in all likelihood the thermal cutoff or the circuit breaker will trip before the pressure exceeds the structural limits of the compressor or even the pinched tube. In the event that the electrical safety features don't operate, that tube will probably be first to fail (uneventfully), but if not, a compressor seal will probably fail before any structural component.

If it was an intentional decision, perhaps they're concerned about the overheating (especially Viair) compressors softening plastic tubing attached to the compressor head. But if so, the safety issue should be dealt with under R8, because it applies to any tubing running in the vicinity, not just the tubing attached to the relief valve.