Because a lot of "instructors" are nothing more than CPLs who can fly an airplane to PTS standards from the right seat.
The pilot of this Cessna was NOT (repeat NOT) a CFI (Certified Flight Instructor, for those non-aviators out there) - he was simply a private pilot, flying with a student pilot. I use past tense because the FAA has pulled his ticket.
I'm figuring these guys were both simply woefully ignorant of the D.C. ADIZ restrictions. They didn't file a flight plan, their radio was marginal, they weren't monitoring appropriate frequencies. It seems unlikely this was intentional penetration of the ADIZ as such action would be essentially 'suicide by shootdown'.