It seems to me that this is quite similar to the requirements set forth in NFPA 58 for the placement of propane storage tanks. A/C window and condenser units are considered to be sources of igntion, as are any sources of ignition. This is defined as:

3.3.67 Devices or equipment that, because of their modes or use of operation, are capable of providing sufficient thermal energy to ignite flamable-LF-gas-vapor-air mixtures when introduced in such a mixture or such a mixture comes into contact with them, and that will permit propagation of flame away from them.

Aaron