How is it not? If the kid is sitting in the basket, playing with whatever toy you gave them, how is that not safe? If you're suggesting the kid would stand, is that before or after you told them to sit in the basket? If it's after, that's disobedience & that would need to be addressed.
Would you address it before or after he fell over the side and split his head open because you jerked the cart to a stop to grab him and he lost his balance?
Putting a child in the basket is not unreasonable, people do it every day and millions of children are just fine. It just wasn't something I was comfortable with.
I would respond that it was a shame the child was injured and comment that the world is an imperfect place.
Unfortunately, you are living in la la land. 100% of the children 100% of the time are not going to obey their parents. Especially an excited child who grabs out at something they really like. Are they being disobedient? Not necessarily. What you and your mom don't seem to understand is that you can keep both your eyeballs on your kids and crap will still happen - that doesn't make you a terrible parent. Don't tell us we're bad parents because we don't keep our children tied at the hip 24/7. Yes, I let my 5 y/o walk a few feet away from me to look at things they like - I can assure you that's no worse than making your children walk next to you with their hands in your jeans.