Because relevant US and EU software copyright law permits reverse engineering as "fair use", blanket contractual prohibitions on reverse engineering (eg, "Licensee will not reverse engineer, decompile, decode, decrypt, disassemble, or in any way derive source code from, the Licensed Software") might not be enforced. See eg SAS Institute, Inc. v. World Programming Ltd. (ECJ 2012); Vault v. Quaid Software Ltd. (US 5th Cir. 1988). These jurisdictions countenance reverse engineering as fair use if conducted in order to achieve interoperability between the licensed software and independently created software.
Most drafters of technology license agreements representing licensors recognize this, and consequently use the following commonly found formulation: "Licensee will not reverse engineer, decompile, decode, decrypt, disassemble, or in any way derive source code from, the Licensed Software, except to the extent enforcement of the foregoing is prohibited by applicable law."
The problem with this clause, however, is that it operates in a purely binary fashion: if the licensee's activities are within the legal contours of fair use, the anti-reverse engineering clause will not be enforced. If they are not, then the clause will be enforced. This type of clause is not much of an improvement over a simple blanket prohibition. No effort is made to attempt to define in advance what the parties would consider fair use.