Regarding the safety concerns, I think everyone has a different level of comfort with risk and kids. I see the list of safety issues lilyka made, for example, and see a few items that I think, "yep, that's a big issue and definitely needs to be used for proper limit setting," but most of the items are like, "eh, that's life."
I just don't relate at all to the "what's wrong with kids doing ____[fill in the blank]____?" when it comes to the response of, "they could get hurt, that's what!" As if "being hurt" is the worst thing in the world. Hurt comes in degrees, and we have to weigh benefits and risks throughout our lives to manage those degrees.
Our kids experience safety risks *everywhere.* I personally don't mind my kids getting *minor* injuries if the benefit outweighs the risk. I do realize that in a minority of cases, minor injuries can result in major crap. Yes, a piece of glass getting in a kids hand could result in an infection, but for the vast, vast majority of such injuries, the glass is safely removed and the child has learned to watch more carefully where s/he is putting his/her hands. Does that mean I want my kids to get hurt? No of course not! We work at prevention, but no matter who you are, prevention will involve weighing risks and benefits of different actions.
Clearly, we are all going to weigh those things slightly differently, and when it comes to minor injury risks, I think that is okay, good, right, and necessary. Most of us probably balance these issues with relevant consideration of what we know about our children, ourselves as supervisors, etc. etc.
I am, for example, all for seat belt laws that require use of proper car restraints for kids...and for severe penalties for those who don't follow those laws. The risk of major injury or death due to lack of proper vehicle restraints is HUGE, and I don't think anybody should be able to just disregard those risks. I even support helmet laws when it comes to bike riding and roller blading. But the risks involved in how high a child can climb on a playground, or where a child can walk holding a hand vs. not holding a hand, and what a child can do in a store is probably best handled on the family level, by people who know the child and the other factors involved.
I am not saying the benefits of being out of the cart are always and necessarily higher than being in. But I am saying, let's not pull out the "but the kid could get hurt" argument as if it is a be-all, end-all. It's one facet of the issue, and it is as nuanced as any other.