Okay, this is the second time now that you have either deliberately ignored my point or failed to grasp it. I will try one MORE time to explain it to you.
You're right, if I exposed part of a buried bumper, I would simply fill in my plug and leave it be. I'm not going to dig a several foot wide hole in my local park. MD'ing would be banned by the end of the week, it would make an unsightly mess, and would be a potential trip hazard. Anything with significant surface area that would mess up the turf if removed stays in the ground without express permission from the landowner - that's the responsible, respectful, and common sense decision. Yes, if I uncovered part of an entire musket in a manicured lawn, I would ask the owner if I had permission to dig it out. It's common sense - don't mess up what belongs to you (ie the park or private residence).
But wait, am I honestly being lectured about "responsibility" from one who ADMITS to littering while metal detecting? Just because so many people are calling you out for your littering doesn't mean you should get all defensive - just stop littering. It gives the rest of us a bad name.
I dug a piece of misc brass that I couldn't ID, and assumed it to be junk. I brought it home and it went into one of my junk bins. Then I saw one posted to this forum, ID'ed as a civil war era epaulet. Went out to my junk bin, found it, and now it's in a display on my mantle piece.
And, imho, on the wrong forum.