NORAD was tracking it over Canada. If concerned about identifying it, dropping it in 50 feet of water off myrtle beach is hardly an improvement. They could have downed it long before the public was aware it existed. This is usually how military ops go - things happen that people don't ever know about and might appear in a document dump or FOI return 15 years later.
Even if canada airspace concerns existed - hitting it the moment it crossed the border and before it became a public spectacle was more than possible.