While I don't speak for the BJCP, I have some personal thoughts on why the guidelines aren't provided on the tasting exam. The goal of just about any test is to verify that the examinee has some knowledge about the subject matter. For the tasting exam, this knowledge is demonstrated in the areas of scoring, perception, descriptive ability, completeness, and feedback. Use of the guidelines during the exam would make it more difficult to determine if the examinee truly has this body of knowledge or if he/she is just regurgitating the guidelines. I have seen judges with perception biases just based on hearing the style category and sub-category such as immediately perceiving DMS in a sample because they know DMS is common for the style, even though it did not exist. This type of bias could be exacerbated with an examinee looking through the guidelines for information to write about.
Descriptive ability would be hard to score as an examinee allowed to use the guidelines would no longer have a need to know how to translate their perceptions into words. He could get all the descriptive terms he needs directly from the guidelines. Scoring and inherent knowledge about the styles would be harder to determine since the guidelines would be used to derive these details. The same could be said for completeness and feedback.
I think the bigger reason that style guidelines are not employed for the exam is that a judge, during competition, should only be relying on the guidelines after all perceptions and descriptive details about a beer being evaluated have been written. When I judge, I don't bother looking at the guidelines until the Aroma, Appearance, Flavor, and Mouthfeel perceptions and descriptions are complete. Once I have evaluated these aspects, I use the guidelines to determine how well the beer meets the style requirements, derive a score, and then write the Overall Impression. During a competition, there is not enough time to constantly read through the guidelines, especially when you are expected to judge a beer in 10 minutes.
One last real world argument is regarding Best of Show judging. If you want to be an excellent Best of Show judge, you need to have a pretty complete knowledge base about all the styles. When you are evaluating 23 or more beers in BOS, you do not have the time to read through the guidelines for style details. You need to apply the knowledge that you have about the beer styles to determine if a particular beer is the best example of style. How do you instill that knowledge into new judges if they don't need to know the guidelines in intimate detail? Granted, BOS usually employs more experienced and knowledgeable judges but those judges started out committing the guidelines to memory.
I think the ultimate goal of the tasting exam is to field qualified and knowledgeable judges. Can this be accomplished if the style guidelines are allowed on the exam? Is there an argument to be made that the use of the guidelines during an exam would result in better judges? If so, then these should be brought to the BJCP for consideration. Most judges and prospective judges I have met usually want to allow the use of the guidelines on the exam to make studying and passing easier which I personally don't think results in better judges.
Since the BJCP has changed the exam format, passing the tasting exam has gotten easier, not harder. If you have basic beer knowledge, can translate your perceptions into descriptions, and can generate a complete scoresheet, you should have no problem passing the tasting exam. The difficulty arises when you aspire to attain higher judging ranks like National or Master.