The pot thing is an arbitrary number
They could as easily have said 10, or 5, or once. What they're really going to look at is whether you pass and continue to pass the drug screen which, I assume, after hiring is random.
As for the fitness, it just makes sense. It's already been explained. Does anybody really believe they're going to work for the FBI or any other similar organization and not go through some type of training. It isn't done that way. Believe me, the physical training isn't that difficult. The FBI are white collar cops, meaning that their physical training isn't as tough as you'll find in say, the NYPD, LAPD, or most other police agencies where the officers are out working the streets.
I am a bit puzzled as to why they aren't planning to use the people for what they hired them for after graduation from the academy. That makes no sense. If they need them now, then use them now. Typical Federal operation, though. Five yrs down the road, things will change so much that the current crop of hackers, away from the computers for years, will be lost.