I must be horribly uncouth. I have put my kids' butt on a counter, I have done it more than once, I would do it again. Never a bare diaper, never a bare bottom. Just a fully clothed kid's butt.
It happens. I have two hands and two toddlers. Sometimes, there isn't a carriage offered, a stroller is a hassle and a half, and it keeps my kid from taking off. Writing a check or signing the credit card slip can be near impossible when being pulled in a couple of different directions. Simple solution, put kid between me and on counter, I hug her (my son is too big to be counter plopped) and sign what I need to sign. As for her grabbing at things. She is like an octopus, it doesn't matter if she's in her stroller, a carriage, in my arms, on the counter; if she sees something she wants, she goes for it.