There is no technical reason why it couldn't be added. However there is political and ethnical reasons to not add that funcionality. There is debatable legality

There is no technical reason why it couldn't be added. However there is political and ethnical reasons to not add that funcionality. There is debatable legality of running a tor exit node. You don't want "Use Firefox, get a visit to the police" to be a headline people can use why not to use FF.