This type of modification would require a lot of 'overhead' within the code. The way the word censor works is that it scans posts for words. Say I censor the word 'jerk'. A user tries to post this:
It is not stored in the database, or the cache, that way. It is actually stored as:
Now, if I wanted to allow users to turn this feature on / off, it would have to store both copies of the message. Thus doubling the size of both your cache and your database.