This is due to the nature of how skill trees work with level expansions.
When you give more skill points to spend to a player, you also make it possible for them to buy more combinations of the lowest level branches of multiple skill trees. They can, for example, buy the lowest level keystone abilities for both a strong DPS character AND a tank, and make characters that are virtually impossible to balance correctly (the classic 'magetank').
We're not thrilled with this, of course. We'd really like for tank classes to be able to tank as quickly as possible. However, some things would need to be rethought before this can be a reality. It's definitely something that will get more thought before we ever raise the level cap again.
Thanks Damion! Very good reason and I appreciate the mention of getting players "going" (in their role) as quick as possible.