Why does the blockchain format accommodate arbitrary text anyway? Who's idea was that? Mr Nakamoto.
If there's currently space for random text, couldn't the blockchain just be smaller instead?
It would be very difficult to prevent arbitrary text since it needs to store arbitrary data. A standard address is just a sequence of 20 bytes. What is stopping anyone from making those bytes an ASCII string?
As soon as you need additional tools to unlock the content, these tools themselves become problematic and can be blocked/outlawed much more easily and with less harm to Bitcoin itself.
How would we restrict the tools? As long as Bitcoin is open-source, the blockchain storage format is openly available. As far as I see it, it would be wrong to try to block or outlaw tools for parsing the blockchain. People should be able to freely analyze its contents.
If we have a problem with content in the blockchain, we need to either find a way to deal with it or learn to accept it. Hiding it won't make it go away.