[ This is obviously a somewhat cynical answer ]
what is the rationale for these limits?
Because the NCAA wishes to maintain the farce that college athletes are amateurs. If they spent unlimited time with their coaches, it would be clear that was in fact their primary occupation and it would be much harder for the NCAA to justify not paying them. By ensuring that the "student-athletes" spend more time on what are nominally learning activities than what are nominally sporting activities, the NCAA successfully protects its business model.
For tennis players, this probably wouldn't matter much as they don't bring in any significant income, but if the NCAA allowed unlimited time for tennis players, there would be no justification for not allowing it for football and basketball players as well, and that would cause problems.