TeX is an interpreter. It can interpret the user supplied scripts.
Interpreters than can run code supplied by user or downloaded from internet are verboten on iOS.
Put it on Cydia?
This has not been true since late 2010, the rule is now that applications cannot interpret scripts downloaded from sources external to the app. Embedding an interpreter in an app is fine.
If this is at all useful, it can render a downloaded .tex document, which equates to running a script from an external source (because TeX macros are Turing-complete even if not convenient).
Well, it's sandboxed interpretation. You can't really do much with TeX aside from maybe reading files from the filesystem.