Demandware (now Salesforce) does not allow their customers to place agents on their backend. This is the only reason why you cannot instrument the backend. There is no technical reason. I highly encourage you to ask them, though, as we have several customers using UEM on the front end who would love to get visibility into the backend.
Also, can I assume you are leveraging the Demandware fastpack? If so, stay tuned, as I'll have a new, much improved version with many cool web dashboards out soon. It's built for AppMon 7.
If you're not already familiar with it, you can find the current version at either of these two locations
and see what we're preparing for AppMon 7.0 in this performance clinic I did with Andi Grabner.