They aren't allowed because they aren't used like they should (in your example), but to mess up websites with walls of garbled characters.
!
I'm sure there's potential for DOS and maybe even buffer overflows or similar shenanigans in there, if your font rendering engine is not rock solid. Good decision
It has been used for years (especially on Digg) to crash browsers/OS, and worse, bricking entire iOS device (would require a restore).
jewgle webm/webp are also blocked for the same reason. They can also carry malware in payloads.
It's always the same story. If your code is too complicated and wants to do too much, (many) more bugs are going to find their way in there. Looking at you, UEFI.
I used it a few times here on Poal in small quantities for novelty purposes. When I did a test with a large quantity of Zalgo text content and it destabilized my browser, I let AOU know that there was a potential for a client-side DOS and he added code to prevent its use and keep the jews from abusing it. I've never seen a legitimate use of Zalgo IRL so this was a good call.
That's true, they're consistently used to forum slide by buttblasted jews and schizos, seen it too many times.