There are many forms of recreation available in public areas. Skating, tennis,soccer, vollyball, football, baseball, frisbee throwing, bicycle riding, and jogging among others. Some of the people who use the areas often leave trash items on and around picnic areas, next to trash containers, and scattered all thru the area. We all have seen this.
Metel detecting is the only recreation being done in these areas that leave the grounds cleaner than before. We take out loads of metal items, in and from on top of the ground and many of us also pick up lots of other trash to remove from sight.
Because of some who don't fill the holes back up, the officials say either that, or the antiquities law as to ban US from the places.
I have seen lots more damage from dogs digging in parks, from dog poop all over green grass areas, soccer players damage to grass, baseball fields with ruts along the base line, and chunks tore out of grass fields from other people using the grounds.
I haven't heard of any of these people in the parks helping law enforcement agencies locate metal items from crimes in the area, just metal detectorists. I haven't heard any other of these park users helping archaeologists find a historical spot lost for years, just metal detectorists. I haven't heard of any of those people finding a lost ring, and if identifiable, returning it to its owner, just metal detectorists.
But they tell us we are NOT WELCOME in these public areas, because of our hobby, metal detecting

??
They should prohibit ALL the other activities also, because they are more damaging to the area, than we are.