Why morally indefensible? Maybe think of it like this:
Suppose your local water supply failed for some reason and you had to queue at a standpipe. Staff to ensure turn-taking at the tap. Someone comes and bribes the staff so as to jump the queue. Acceptable behaviour?
Or suppose your local health service supply failed for some reason and you had to queue at a hospital. Staff to ensure turn-taking at the a&e. Someone comes and bribes the staff so as to jump the queue. Acceptable behaviour?
Or suppose your local schoolteacher supply failed ... oh, you know the rest.
Can you, or not, see how private health care and education are morally equivalent to bribery facilitating queue-jumping? (Particularly given the public provision we have organised as a society.)
Sure, health care, education, public goods in general, may need to be rationed in times of scarcity of resource. But to ration by price rather than need is just obviously wrong. It's corrupt in the same way - quite precisely - as bribery. No?
Of course you might argue - along with Hayek and Friedman and all the rest - that the rich should be allowed to bribe their way to the front of any queue, for the sake of economic benefits to be gained by market allocation of scarce resources. You probably will, some of you.