Ague all you want about what you think nursing mothers should do when out in public, it doesn't matter. She is protected by LAW to nurse wherever she and the baby are otherwise allowed to be. No, she wouldn't be allowed to nurse at a stag club because women aren't allowed there. If it's a restaurant that has a no child policy, she wouldn't be allowed to nurse there either. However, a grocery store, a public pool, the local diner, and church are all places where she is allowed to be with her baby.
It doesn't matter if you think it is appropriate. It doesn't matter if she chooses to go to another room, cover, use the two shirt method, use only one shirt, feed in a baby carrier, or have her entire chest exposed. She is protected by law and has every right to do that.
If you choose to say something to her along the lines of covering up or leaving the vicinity, unlike asking someone who is sagging to pull their pants up, you wouldn't simply be considered rude; you would be violating her rights and that is illegal. If you work at, manage, or own an establishment and ask her to cover up or leave, prepare to have a bunch of nursing moms in your establishment to bring to your attention the fact that what you did was illegal.
If you truly want to understand why some moms do a certain action while others don't, asking questions is fine. And you're welcome to have your opinion; nobody is trying to stop that. All we want is to be left alone when we're feeding our babies. We aren't out to seek attention, unless YOU want to make a big deal out of it. Breasts are for feeding babies, and babies should be fed anywhere they happen to be when they get hungry.