A beaker full of death wrote:
One point of view would be the prime directive is subject to constant change.
In TOS it wasn't some nebulous philosophy. It was a codified military regulation. General Order #1. It had specific rules for what to do and what not to do. McCoy and Kirk quote part of it in Bread and Circuses.
The problem is TOS was inconsistent with the use of the Prime Directive. If you look at the series by production number this is clear:
"The Return of the Archons" (22) referred as Non-interference Directive to by Spock
"A Taste of Armageddon" (23) ignored to the point the Federation Ambassador insisted on beaming down
"Errand of Mercy" (27) We are about to go to war with the Klingons so we will ignore the Prime Directive for now.
"Friday's Child" (32) Pre-warp culture offered aid because the Klingons also want it. Prime Directive is not even referred to even though it should apply.
"The Apple" (38) Spock again refers to Non-interference Directive and Kirk uses same argument as in "The Return of the Archons"
"Bread and Circuses" (43) quoted at length
"A Private Little War" (45) referred to and then discarded due to interference by the Klingons
"A Piece of the Action" (49) - clean up a mess we made
"Patterns of Force" (52) We fix the mess made by John "Darwin Award Winner" Gill
"The Omega Glory" (54) Kirk's quote conflicts with his behavior in "A Taste of Armageddon"
"The Paradise Syndrome" (58) Neither Spock or Kirk make any reference to the Prime Directive though they should
"For the World is Hollow and I Have Touched the Sky" (65) Again the Prime Directive should come up but doesn't
"The Cloud Minders" (74) Prime Directive? We need that Zenite to stop a plague so forget that noise.