I was taught that prepositional phrases were essentially "expendable" components of a sentence that, while referring to the subject-predicate, are 'not really there.' In the context of the following sentence the author uses a parenthetical phrase to refer to the 'thought' presented by a prepositional phrase (the 'thought' being "making marijuana a cash crop"):

In light of the potential for the government-sanctioned sale of marijuana to end California's economic woes, Governor Schwarzenegger is calling for an in-depth analysis of the obvious and real fiscal benefits of making marijuana a cash crop -- a process that would require legalization and taxation of a substance currently listed as a Schedule I drug (i.e., a drug considered to have no medical uses) on the federal roster.

The structure of this sentence opposes my understanding of proper mechanics, in that the parenthetical phrase beginning with "a process" should necessarily refer to the original subject-predicate "Governer Shwarzenegger is calling," and not, as intended by the author, to the prepositional phrase "of making marijuana a cash crop." Am I mistaken in my evaluation that this sentence is written improperly? Furthermore, how could the author have extended the 'thought' presented by the parenthetical phrase "of making marijuana a cash crop" acceptably, in the manner intended?