It would depend on HOW they are walking away. If they are making a show to the kid of walking away, with an "I'm not buying into your tantrum, get over it and follow me" and not really walking away (staying where they can easily see the kid and not putting the kid, or others, in danger) - I've seen this and it doesn't bother me one bit. It's working on correcting the bad behavior and an attempt to stop, rather than escalate, the behavior. Most of the times I see this, as soon as the parent starts to turn a corner, the kid realizes they're about to be left behind, shuts up and runs after the parent and the tantrum is over.
If the parent is just completely ignoring the kid and really is walking away and is leaving the problem for others to deal with or leaving the kid in danger, THAT I have a problem with.
Pretend ignoring and actual ignoring are two very different things. I have said something, once, to a parent who did the second. The woman was in one place and the kid was several aisles down, no longer throwing a tantrum, but instead sobbing because the mom just left without a care in the world - I told her that she needed to watch her kid and not leave it to the rest of the customers to deal with and that if she didn't go back to where she could see her kid, I'd get the store manager to call in an abandoned child. She gave me a nasty look but went back to where the kid was before the kid got grabbed up by some stranger.