Couple thoughts here. First, giving a 16 year old kid with awesome talent a pile of money does not guarantee that he will have the same desire and drive as Joey Votto to become great.
Secondly, when teams schedule fall instructional league programs, or in this case, mini-camps, they are not cheap for the organizaton. They are major undertakings that require the organization to pay coaches, pay for housing, feed the players, insurance, all that stuff. When players, particularly players that were given a $2.5 million signing bonus, elect not to attend, it usually does not make the organization very happy unless there is a good excuse.
Staying with the comparison to the accountant coming in at 7:50, I would counter that this is more like the company scheduling a special training seminar for some of its employees prior to the start of the work day and hiring people to come to the office and train said employees on using the software that they will work with in their jobs, and then having one of your accountants, maybe the one that you invested the most money in for travel expenses to relocate him to your city from another town, not attend the training.
Rojas, Means, Buchholz, Torreyes, Sierra, Duran, LaMarre, Barnhart, Vidal, Hamilton, Waldrop, and Arias (and some I am forgetting) all were there. Good for those guys for getting an early start on their seasons and hopefully that will help keep them one step ahead of the opposition all the way through spring training and then after the bell rings.