Because it creates unjustified Media Hype. This in turn leads to governments "having to do something" even if it is a non issue.
In your case at 2500 over LAX it is doubtful you could create any problem in your Cessna...a 747 has a take off speed probably higher than your Vne, you are on Radar whether in the mini-route, special flight rules area (squawk 1201 and Socal do watch/monitor the AA Coms), or the shoreline/hollywood park routes. Worst you could do is make a smudge on the runway causing it to be closed for 30 minutes.
I can fly o/h Gatwick in the UK if I want, not to mention most of the big UK airports other than LHR.