If I remember correctly, this was part of a fantasy that Flash Thompson had while hooked up to a machine by Mysterio. In it, he was a hugely popular superhero, and Spider-Man was his sidekick. Things were otherwise pretty much the same, but one of the main differences was that JJJ was NOT a powerful newspaperman, he was an obsessive stalker/heckler of Spider-Man who spent most of his time following Flash and Spidey around and ranting at them. (This was not spelled out as such, but it was pretty heavily implied.) Nobody took him seriously, and, in fact, people cheered after Spidey punched him out. Clearly, this was wish-fulfillment for Flash - as a Spider-fan, he'd probably gotten pretty ticked off at JJJ over the years. So while it may not have been in character for Spider-Man to do it, it was perfectly in character for Flash to IMAGINE him doing it. (Not to mention personally satisfying for me to read.)