I wonder if it has anything to do with a prohibition on fully spelling out G_d as some people do. I'd like to know any biblical references pertaining to that - gotta have a "controlling legal authority", don't cha know? If there is a basis for not spelling out G_d, then one could easily make the leap that "X" would also be appropriate for His Son. Personally, I spell them out.
The thinking is, that it is commanded that the name of the Lord not be destroyed. Therefore, writing the Name on any ephemeral substance (and what's more ephemeral than a computer, which writing disappears once power is removed?)* violates that law.
*Of course, one could counter-argue that the posting database still remains. One could then counter-counter-argue that 1. there's no guarantee that the database will remain forever, and 2. it doesn't matter whether or not the writing is a copy - any instance of the written Name must be preserved.