I have used many different airports of entry into the US, including MSP several times, and have never heard of a landing permit requirement.
If you don't carefully follow the expected protocol some inspectors snap and begin to bully. Not giving proper notification, taxiing to the FBO before the Customs ramp or even opening the cabin entry door before the inspector has arrived at the plane can get them started....doesn't take much.
Not saying you did any of this but your experience sounds odd as there is no required landing permit at MSP or any other US public airport I know of.
Word to the Wise: avoid clearing US Customs at any S. Florida airport of entry...always very difficult even on a good day. I'm convinced inspectors who can't get along with others are sent to work there. ALL pax and luggage off, dogs searching the plane, interrogation as luggage is searched, etc. The slightest snag will cause a very long delay. Get yourself an Overflight Permit and clear ANYWHERE else.