Yes, you can kill a feral hog with a .22 if you shoot him in the ear or just behind it. Don't even think about it anywhere else. If you should wound one, especially a boar, they often will charge you. I have been charged twice, once after wounding a hog with an arrow and another after wounding one with a .223. Having a .357 handy when archery hunting hogs was a good idea in my case. I had a buddy who I was archery hunting hogs with that got charged after wounding one. He ended up in hand to hoof close quarters combat with it. I could not shoot the hog with my pistol for fear of hitting my buddy. He drew his side knife and stabbed the hog to death. He was OK other than a few scratches and needing a change of underwear. His bow got busted up some. Good thing it was not a real big hog.
It does seem a paradox that they are such a nuscience critter for landowners, but few will open their land to public hunting. However, the comments by many here already made are the truth. Landowners would be taking a big chance letting just anyone come on their property to hunt. The general public will tear up God only knows what, such as fences, farm equipment, feeders, may shoot livestock, shoot into buildings, shoot at farm equipment, etc. Many yahoos will shoot at any rustle in the bushes they hear or see, never knowing for sure what they are shooting at. Others never consider where their bullets go if they should miss. And as mentioned, once you let them on your land they think they have a right to access it forever. I would like to think that most here are much more responsible and respectful of other's property, but many of the public would do much of the above. Sad, but that is reality. Many landowners will let you hunt hogs on their property IF they know you and trust you to act responsibly.
