I do a lot of school photos and I'm always aware of removing moles/birthmarks by mistake. I do a lot of B&W conversions and often it's difficult to distinguish a small mole from a spot or even some food etc around the mouth of a smaller child when it's in B&W
I often refer back to the colour original to try to determine what to remove but, believe it or not, it is often difficult to decide.
I have a company policy though that if any parent comments that a distinguishing mark is removed by mistake we will re-print the photo correctly for free. I once removed a tiny mole from a shot of my gf and she said straight away 'but without the mole it's not me' - lesson learnt!
I do spend a lot of time removing spots, runny noses, food and so on but that's just to make the photo look nice for the parents and justifiable in my book. I do not do the fake, smoothe skin airbrushing that some photographers to do portraits - that's just too fake and I hate it.
I have removed cold sores, excema, scratches, bruises, black eyes and so on and have had many comments of thanks from parents for not changing their child but just smoothing out temporary skin conditions. However, in this case that photographer was absolutely wrong to remove such a birth mark (I wouldn't dream of doing so) and should be taken to task over it and a reprint demanded. If they refuse take it up with any professional body he/she's a member of as it's a big error in my view!