Tweeters operate significantly above their resonant frequency, so the air volume behind doesn't matter - whereas for bass drivers it dominates their loading, damping and powerhandling.
As a result, some tweeters are sealed, some are open - i.e. vented to the rear - and this choice is based on manufacturer's design considerations. Generally the latter type need to be in their own chamber, but only to protect the diaphragm from the internal cabinet pressure developed by the bass unit.