I mean, how is that different from reneging on something like Sleep Clause, or Evasion Clause, or... whatever else, really?
This isn't a case where you can hide the fact that you're cheating: the only way you can hide the fact that you lied about your Tera Types is by not terastalizing. After all, as soon as you use it, the opponent will know you lied.
The consequences of doing a Terastalize you lied about are the exact same as the consequences of Sporing multiple enemy mons or suddenly busting out Double Team or something else illegal.
I'm not saying that a "reveal Tera" clause is the correct choice, but I don't find this to be a compelling argument against it.