I'm not sure how its a gray area at all, but maybe thats just me. Salt doesn't grow naturally in the wild and salt and by-products of salt are illegal bait. They tested the soil and there was salt present. If you want to be legal 1.) don't hunt anywhere near where there was salt, or if you do make sure its all gone. It won't be hard to persuade a judge that salty soil is a by product of the salt block being there. It would be the same as going out and sprinkling rock salt on the ground...just because you can't see it, doesn't mean its not there.
Do people do it all over the state...Yep...just as illegal. The high profile this deer was given put a lot of eyes on it, so of course there will always be extra scrutiny.
I honestly could give a crap less though...I don't deer hunt but I have no problem with trophy hunting. It never ceases to amaze me how jealous/greedy people get when it comes to trophies. In this case, if the guy loses to the DNR (Which he probably will if the salt was found close to where he was) then he may have a case for illegal search and seizure, but even that would be a stretch.
How soon after he bought the property did he shoot the buck? Is it possible anyone else toured the property with interest in buying it and saw the salt licks? Maybe they got outbid on the land and told the DNR about the salt? I guess the court rulings will tell, but unfortunately, it doesn't look good for this guy.