Call BMWNA immediately and ask to talk to a customer service manager. Not a regular telephone monkey, but the big cahuna - basically just ask to go up the payscale until you reach the top.
Then tell him/her about this experience, and how displeased you are, and how much you were shocked that a high end brand like BMW can use Hoffa-style tactics and hound and investigate their customers.
Tell them that in order to win a customer back, a business usually has to go an extra mile, not scare them into submission. That never works, as evidenced by your current telephone call.
He will probably try to wriggle from under the responsibility saying that they don't control the dealers, in which case you can tell them that the local paper might be more receptive to hearing this story.
Finally ask to take the customer satisfaction survey and give them all 1's.
Afterwards, open up a complaint with the BBB.
At the very least, also send a letter/email to Roundel - it will get published.
09 M3 Sedan ED
12 MINI Countryman S All4
93 Miata LE