In a situation like this, the worry isn't (for the most part) about people wanting to "cheat" and intentionally bring in a hot gun. The main worry should be people who do not know what their guns shoot. MOST airsofters actually do not know exactly what their guns shoot - only what is quoted to them for similar guns in similar configurations, but each gun is different.
Another potential exception to the "stock internals" rule: I understand that the reason for this is to keep velocities low to prevent injury. However, one of my AEGs is highly tuned for velocity consistency, increased trigger response time, and reduced power consumption. It shoots 314fps consistently with the hop up off and 304fps with the hop up properly adjusted for a straight trajectory with 0.28g BBs. It's a hair over the 300fps mark and has not hurt anybody in the 2 years I've used it, nor will it ever reach a point where it hurts somebody. However, it automatically breaks the rules due to the "stock internals only" rule.