Birdsgottafly makes a great point. A distinction needs to be drawn between PND and a perfectly justified reaction to how UK society treats mothers. Please note I'm not saying PND doesn't exist!
A friend went from satisfying job and happy pregnancy to a horrible last few weeks (sunny weather, active kicking baby = no sleep) to a traumatic birth, a horrible ward experience, and then home. Only she can stay at home (maternity pay is only for women, obv) and she had a baby that refused to be put down or left in any way - screaming in a way that I wouldn't believe unless I'd witnessed it. She essentially had a baby attached to her for about four months. Her husband works long hours because they've lost an income and her only way out was to places like Rhythm Time where they talk to her as though she's six. It sounds bloody horrible, to be honest.
She confessed to the HV that she was finding it hard, but all they wanted to talk about was PND. She doesn't need ADs because she's not depressed - she's having a perfectly sensible reaction that any human would have after four months with about three hours of sleep a night and a life that is so suddenly out of step with what it's always been.
There's an unacknowledged 'gap' I think, between PND and this sort of stuff, brought about by the fact that we as a society project this idea of motherhood as perfect and easy, when it's not the case at all.
I think, OP, that a lot of women are told they have PND when they don't (some do, of course, it does exist and shouldn't be denied) because it's the only way society can cope with mothers turning around and saying 'do you know what, this is shit'.