By the description of Pounce, the creature is not limited to natural attacks. It gains a full attack at the end of the charge.
What you have here is that the Lion Totem ability just very powerful in certain circumstances. The Barbarian would be able to get two off two good hits raging and power attacking as he reaches his target (before the other spell caster kill everything else).
This will be most useful at level 6, though will wane after level 11 as the other classes (wizard, druid, cleric) starts commanding the battlefield.
If you claim the totem barbarian could only use slam attacks then he would be provoking unless he had the unarmed strike feat. Fortunately, a barbarian can't multi-class with a monk, otherwise the pounce would be extremely powerful.