Question

why were the British compelled to allow the tribal people to carry on shifting cultivators in some forests?

Answer 1

The British wanted tribal groups to settle down and become peasant cultivators, because as settled peasants they were easier to control and administer.

So, the British compelled the tribal people to carry on shifting cultivators in some forests. The British also wanted a regular revenue source for the state.

So they introduced land settlements, that is, they measured the land, defined the rights of each individual to that land, and fixed the revenue the farmer had to pay to the state.