It kinda is. As far as pay scale goes, the easier you are to replace, the less you make.
Cart Attendent and Cashier are at the bottom. That's because you can pretty much grab any TM off the floor, give them 20 minutes of training, and they'd be able to do the job with minimal supervision. These are easily replaceable positions.
Next is Sales Floor, GSA and such. They require a little more experience which is why they are scheduled for multiple training. Most SFTMs wont argue that pay potions, but I know there are GSA that think they should be making GSTL pay. But in reality, they are almost as easily replaceable as a SFTM. Mainly because the GSTL does the exact same thing so if push comes to shove, they would just take over.
Next is Logistics. These TM's are a lot harder to replace than the ones before. Mainly because it takes quite a while to get all the PDA functions, pushing, and instock skills necessary to actually. You could easily take a BR and have him get carts. But you couldn't take a CA and have him pull the CAFs in a timely manner.
Lasts is things like PA, Signing Specialist and CO where there is usually one or two TMs that do that role.
I do think everyone should get paid a little more, but I do feel they pay by how much the job is worth, at least to the store.