Thanks cosmo, that makes sense to me. INOP can mean a motor failure (in which case you can trim manually) or a physically jam (in which case you can't) and to cover the latter they are recommending a lower landing flap. Whereas its assumed that runaway can be halted by the cutoff and then trim is definitely available, hence no flap restriction.
Personally, I'd have been tempted to state that the flaps 15 applies only if manual trim is found not to work, so allowing the full landing flap to be more available. But maybe most of the failures are jams - I'm having a hard time believing that though. Or maybe just a case of Keep It Simple?
Always interesting to se how other people have handled failures - often you find differences in philosophy, and sometimes something you can learn from - or steal shamelessly, if you prefer!