Well, we know they are using slave labor to clear the road and apparently be used as sacrificial lambs should the Mallers be attacked by zombies.

I really really have no idea why the Eastern Bay option is so attractive to everyone. Imagine you are a convicted murderer, you have been locked away in a hole in Eastern Bay for 18 years. Suddenly, you're free to roam about the city and do whatever you want. What would make you even think Eastern Bay is a place you'd want to go back to? So I don't think Eastern Bay is getting any slave labor done. Besides, electronic doors in jails would be pretty much inoperable at this point thus negating much of the safety provided by the prison.

One place to get the slaves would be the mall. Enough people have seen Dawn of the Dead that they probably thought going to the mall would be an awesome way to ride out a zombie apocalypse. They just didn't count on the mall getting over run with ex-cons who are aggressive and willing to do anything. I also wouldn't be surprised if they attracted people while they were on the move. A force of bad ass people carrying weapons (assuming they stole what the prison had in its armory) would seem like a better situation than most people are in at the time of the outbreak.

Their mall is a strip mall. Strip malls aren't as porous as a regular mall. There hardest part would be to blocking off the entrances to the parking lot and the back loading doors to the different stores. So if you can block off a perimeter, it's relatively safe, but as Durai pointed out there are many drawbacks. They had made a look out Tower, the one that shot at Saul on the roof.

The Mallers had to get their slaves early on when there were survivors...you know...alive. If they tried to get them at this point, they'd be running into people who had survived for over 3 months in this hell, who would have to be pretty well prepared and very not inclined to listen to another group of survivors. We've seen nothing but hostility between survivors when they first meet because remember: All the passive ones are dead.