Yeah, I get really sick of non-parents flaunting their snide preconceptions of "If all parents weren't lazy, self-absorbed trash, then all children would be perfect little angels in public." It gets really... really... really old. You have no idea, as a non-parent, how often these attitudes are encountered when you become a parent. You have no idea how clueless people can be when, for example, your kid throws a fit in public because you're disciplining them... and somebody yells at you because they think your kid is throwing a fit because you're not disciplining them... For me, it's a huge lesson in how sickeningly judgmental and intolerant people can be.
Had to get that off my chest.
As for the leashes... I have very mixed feelings about them. A kid is very deeply effected by their early experiences. I can imagine use of these things having weird interactions with a kid's feelings about boundaries to their personal freedom and confidence. Depending on the person, I can see them growing to have amplified rebelliousness or dependence in their personalities. There is also something fundamentally offensive about the idea.
On the other hand, I can see how it could be incredibly helpful in various situations. Being the perfect parent all the time simply isn't possible. People have stress limits. A perfect saint can become an abusive parent when those limits are crossed. People have awareness limits and only two hands. It may look irresponsible to an outsider when a parent cuts corners for their own convenience. An outsider doesn't know that parent's situation.
Being a parent is a huge responsibility. Yes, both non-parents and parents know that we should always strive to do the right thing that is most fair and beneficial for the child. However, non-parents typically haven't emotionally absorbed what failure as a parent means, which is a dead or extremely fucked up kid. A responsible parent will do the ideal thing whenever possible, but will also cut corners when doing so reduces a significant likelihood of catastrophic failure. This balancing act is up to the individual, and is why parents deserve a lot more tolerance and understanding than non-parents like to grant them.
Personally, we've never used a leash. We have used one of those hands-free carriers on rare occasions. Usually we just do things together as much as possible, and running away results in me carrying or timing-out the impulsive kid.