...is zero except for the pressure drop of the checkvalve. So the strength of the retainment can be similarly wimpy and still be safe.
But if you convert to HPA, then pressure acts on the entire frontal area of the to create an astronomically higher 100s or 1000s of pounds of force trying to launch the valve out the back end of the tube and (potentially) into the shooter's smile.
Hence the required strength of retainment needs to be similarly astronomically higher.
If you want to keep smiling.