Not just Scientologism. Shouldn't we be reexamining all tax exempt organizations that promote a religious belief as their sole claim to tax exempt? Run a soup kitchen, great, soup kitchen is tax exempt. Run an empire with a soup kitchen, the empire should not be tax exempt- true for scientologists and Christians.
The original reason was because they used that status to help people (i.e. supply food/help for the poor and elderly). It was from a time before things like social security/etc. and many elderly lived in extreme poverty. Many social functions (that churches received the exemption for) are now handled by many other social services.
They no longer need the exemptions as it now just allows them to buy up land/property