Why? Even without touching the tree, a wild koala can become distressed if humans approach too close. Unhabituated koalas often wake up, become very rigid and will sometimes climb higher when humans approach. So we have formulated this rule as a basic safe and respectful distance.
Interestingly, a recent scientific study (1) has found that captive koalas subjected to people at close proximity (5 metres) do experience higher levels of stress than when the people are further away. Wild koalas are likely to be even more sensitive, so our self-imposed 10 metre limit is justified.