How does a maximum power point charge controller regulate the charging current, when it always draws the maximum current from the panel?
Good question. You are right that there is a contradiction there. How can you do maximum power point tracking if the load is not able to accept the maximum power? For a grid tie inverter, it is no problem because the grid can accept all the power that the solar array can produce.

But what about a battery? What if the battery is charged already, or if the maximum power would result in too high of a charge current? The way this contradiction is resolved is that the MPPT charge controller for an off-grid system will do MPPT only when the battery can accept the maximum power. At other times, it will abandon the MPPT protocol and function as a normal charge controller.

In a typical off-grid system, the battery will be large enough to accept the solar array's maximum power output during bulk-charging, but once the charge controller transitions to constant voltage, the maximum power will not be used.