10. Johnson is really a cop. As cop, fed or corp security his job is to get the runners into a position where they can be apprehended with minimal effort and danger. He'll give them a "sneaky sneaky no-killing infiltration" job on a strict time table that requires them to leave the heavy ordinance behind. Next thing the team knows, they've wandered right into an ambush against overwhelming force and they're caught without their best toys for such a situation.
OK, gonna play Devil's Advocate for some food for thought here for a moment, so don't take this personaly.

I know it's just a game, but one would need to be careful over this one becasue of a single word: ENTRAPMENT. Mr. Johnson is specifically hiring the runners to do something illegal. He is the one making the offer, telling the runners what to do/go after/etc, and has initiated the job. Johnson is now directly responsible for and helping the runners commit the crime in question...thats called conspiracy, also a crime, and even more reason why Mr. Johnson IS Mr. Johnson in SR. It's the same reason why all undercover officers must be very careful and not initiate any unlawful activity; why undercover officers posing as prostitutes must let the johns initiate things first and vice versa. It's also why undercover officers working on line to catch predators have to let them make the first move.
Feds would know better as they still have to obey the law. This would also likely torpedo their career and call all of their other cases into question. Most Feds wouldn't risk that. Even compaines that provide law enforcement have to abide by the law, its part of the contract, and any lawyer would eat this alive. Corporate security, why? Why go through such lengths? If a corp wants to get back at runners, it's easier to simply to metaphorically frag them and be done with it.
Why not just bust them at the meet? Why not just get them seperated and tail them to bust them later? Why go after them when they are most vulnerable instead of at least semi-ready. Why risk unnecessary damage to property and or death of officers? Besides, how many runners are dumb enough to go in without "Plan B" (aka the BIG very-lethal guns for just such a situation).
Now, I admit, that crooked cops/etc could do this, but the runners had better be "killed while trying to escape" or something, otherwise Johnson risks being discovered. It's still a viable plan and plot hook, but sometimes players are quick to pounce on such things and could see a GM pushing this more as a screw-job, especially if the crooked cop angle is pushed too far. It's a delicate balancing act. A cop's job is not to make people commit crimes, but to prevent and or apprehend them when they do.
And like I said, just playing Devil's Advocate. But +1 for the rest.