I am curious about where in your Ops Specs this is specified. The FOM is written by the company and may contain that language. And unless you are using a performance vendor to calculate takeoff and landing weights, I'm not sure what "safe altitude" is referring to.
It looks like you are flying the Lear right now, and if I remember correctly the AFM states three criteria for calculating the max landing weight. The maximum structural landing weight, the maximum weight allowed for the runway length, and the maximum weight that will still allow approach and landing climb gradients (2.1% and 3.2%) to be attained. The most restrictive of these weights is controlling. That's the Part 25 requirements, so if you exceed one of these limits (which is usually the approach or landing climb), the airplane is being operated outside of its limitations.
So the 2.1% approach climb requirement is just to be sure you are operating the aircraft within it's limitations. To tie this in with TERPS, there are companies that do an airport analysis and create single engine procedures for specific aircraft that will deliver you to a safe altitude in case you bag an engine.
For the AIM question, the way I understand it is that the 40:1 plane (152' per nm) is the standard plane that no obstacles may penetrate. Climbing at 200' per NM as specified in the AIM gives you a buffer between the airplane and the obstacles that extend to the 40:1 plane.
Hope this helps. Also wanted to point out that this general topic has been discussed previously. I can't remember the title of the thread, but Lead Sled has a few detailed posts that are very informative.
