I think Allen is arguing that, if the beached officers were either on full pay and forced to accept postings by law, or no pay and forced by poverty, they would

I think Allen is arguing that, if the beached officers were either on full pay and forced to accept postings by law, or no pay and forced by poverty, they would not be able to reject commissions. They would thus be forced to accept adverse postings that were bad for the navy as a whole. They were able to reject commissions because they weren't technically employed by the navy while unassigned. Officers could turn down a posting and still draw half pay, and in fact they kept their half pay during retirement.