Usually the non buyers fall into two categories:
1. Don't make enough IPH to buy weapons. Probably leveled up playing the Jobs too much.
2. They don't get, even though it's on the screen about what you get for each henchman, that you need exp. Seen many 500 plus mobs attack me with less than 2000. When I look at their profile it's like 0 on explosives.
There are silly sorts that hate losing disposable purchases as well I guess. Their loss and your gain if attacked or attacking them.