Actually, the best method for avoiding union interference is to not treat employees like shit, thus removing incentive to join a union.
Agreed. Companies of Northern/Midwestern states figured that treating their employees with respect was the best and cheapest way to limit (and prevent) unionization.
Unfortunately, most of those states, save Ohio in the Midwest, have been overtaken by political interests that ramrodded the Southern way of business. To undo that will be like Hercules cleaning the Augean stables.
On the other hand, I hear Walmart does quite well with their "burn anyone who so much as mentions the word 'union' alive" policy, so I could be way off base.
That's not so much Wal-Mart but a prevalent Southern mindset for any company wishing to do business in the South (or in sufficiently Southernized states like Michigan, Indiana, and Wisconsin). Volkswagen's intent to form works councils was met with political interests that intimidated enough people to vote against it - out of fear. Similar unionization efforts with other employers have received the same "kill it with fire, no matter how much it costs!" philosophy.
The Southern idea is that every resource on Earth and above must be expended to kill off unionization, then follow it up with an employers' union - like a staffing agency or temporary labor service.