I agree with you, and what seems to be the majority. I don't use little potties. I hate 'em. Used them when training my first two (boys) and then realized that, in my experience, you are creating more work, because then you need to "retrain" them to go on the regular toilet.
I have made it more than clear to the parents what my expectations are with potty-training. The child needs to be aware of what is going on. Taking a child to the bathroom every 20-30 minutes and getting accidental pees/poops is not training them. They need to make the progression from totally unaware, to "I peed", to "I am peeing", to "I have to pee". I understand that this happens differently for every child, and am willing to work with parents.
But I am very clear that I cannot take them on a schedule, nor can I spend 5-10 min in the bathroom with a child.
Honestly, I wouldn't use the tiny potty for sanitary reasons. That is how I would discuss it with the parents.