The reason for collared geese VS just a leg band is for local tracking via a spotting scope with collars. That is what they were doing with the metro goose population to watch their movments to determine their main flight paths. As I recall they also did some gps tranmitters too. Leg banding only is for a different scientific reason.
Now that being said, specifically targetting banded geese is why the Govt waterfowl guys tell the volunteers to keep mum where they do banding. Zink is a prime example, as reportedly he was tipped off where in OH they had banded and he then heavily hunting those areas years ago. I have no idea if he still does that. I hope that he has stopped.
As for collars, my feeling is that anyone using a rifle (which is illegal) to shoot the geese when caught should be an automatic loss of equipment, vehicle, and loss of hunting privelages for 5 years. If the collared geese are shot with a shotgun(legal) during regular hunting, who cares.
I do not know what is happening the last few years, but 5+ years ago I regularly heard about hunters using high powered rifles to go after collars and banded Snow geese, especially in MO. Reports like 20 geese in the bed of the truck with 1/2 having collars or bands and no shotgun but a couple of rifles with scopes in the truck type of thing. Also they were tresspassing too. I am steadfast against them systematically flaunting the law by these poachers, thus why I take a harsh view of them. I view them like deer poachers shing at night in mid Oct.