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.
You assume they're being arrested for THAT job. While they could be, SR is a distopia with rampant corruption after all, the Johnson is merely giving them a job that doesn't exist with specific requirements that put non-lethal weapons in their hands making them much easier to arrest. It's the cops basically tellign them, be at this completely safe location where collateral damage is minimized without your big bang bangs so we can nicely take you into custody in a controled and safe manner. It's a big sting operation, an outgrowth of the current "Wow! You have won a boat/season tickets/a new car" stings cops and feds run today.
Secondly, Corp security doesn't have to worry about little things like entrapment. They don't have to play by those rules, they have their own and can break them at will.
Third, entrapment is only for those with a SIN who you intend to actually let have a trial.
As for why not to grab them at the meet, you don't know what they'll be carrying at the meet. You don't know how much legwork they will have done about you or the meet. Beyond that, I rarely, as a player or a GM, have the whole team at the meet. It's typically handled by two, maybe three people, with the rest of the team somewhere nearby ready for trouble. The meet could become a shooting gallery with big guns galore and you caught right next to the "bad guys." Giving them a job where you choose the time and place plus their basic equipment loadout means you've controled the situation and made it more safe than just getting them at the meet where they could be packing serious hardware. Additionaly, they'll tell you pretty much exactly what their plan is for the job if you make that a requirement for employment.
It's about exerting as much control on the situation as possible to minimize casualities.
By way of a real world example of police doing this. 15 years ago, I roomed with a vice cop in a little town in the carolinas that was almost at the half-way point between Miami and New York on a major highway. literally tons of drugs flowed through that little town (population under 2,500). We had mob and drug gang involvement, prostution and street gangs (again, SMALL southern town) just like the big cities, all fighting over turf and drug routes. I was brought on as an armed reserve officer (also known as the FauxCops back then) because of my assocition with the drug sniffing dog and because I was a new face; I could go out and pick up the "girls/undercover cops" from the prostution stings and get them off the streets without arousing any suspicion if necessary. The cops would hire a deniable asset (usually some "new guy" who was unknown at the beginning) and make ever increasing buys from a specific set of dealers until they were trusted. This built the case against the dealers. Then the trusted buyer would make a big buy, something huge, something that required the stuff be brought in. The buyer would make some demands, like the location of the delivery and when the delivery was made, the cops were already in place to grab everyone. They were happy to get the drugs, the money and a few dealers, but what they really wanted was the drivers, the guys who knew where it came from. But they controlled the situation as much as possible by picking the time and place and, to a degree, the kind of "security" they had to deal with by how much product they ordered.
