leaving a sticker is definitely not tortious interference with business relationship
If I was the business owner I’d let the courts make that determination.
“Tortious interference . . . may occur when a first party’s conduct intentionally causes a second party not to enter into a business relationship with a third party that otherwise would probably have occurred.”
Leaving a sticker is definitely tortious interference, if the sticker is offensive to a broad swath of the population and likely to be interpreted as an offensive stance taken by the business owner. It’s for the courts to decide, but as a juror my inclination would be to hold the vandal liable not just for the cost of scraping off the sticker but also for economic damages based on lost business and the maximum legal punitive damages based on the obvious malice. This woman is lucky to get away with just losing access to a good restaurant; she could easily and justly have lost everything she owns.
How about if you left a sticker on the window saying "NO NEGROS ALLOWED"?
Could you be prosecuted for that?