Well, it's always a YMMV situation. I'm curious- what do you mean they "DO NOT" do it? What do they say to you and what is their explanation?
In any event, you can just dumpster dive. All Blockbusters use lockable cases as per corporate mandate. The managers have absolutely nothing to gain by denying you as they end up just throwing them away. Some try to sell some on the side, but they never have enough takers to offload the hundreds of cases they end up with. The only catch is that lately Blockbuster corporate has been having troublesome negotiations with the Hollywood studios, so they haven't been sending out movies in the huge quantities that they used to- ergo, stores might not have as many cases as usual.
Basically, if a manager freezes you out, ask them if they need to borrow some astroglide and a pair of forceps because they must have a serious stick up their ass.