Mirikon: is there a specific rule that I'm missing somewhere? The rules in 4th edition don't really read that way to me (that is to say, I read them as: here are the guidelines for making balanced spells, not: you can do anything you want, but we don't recommend you go outside this).
The restriction on Permanent spells is written as: Only X, Y, Z, spells should be permanent. It doesn't really seem like it is giving allowance or specifically ruling that otherwise is possible...
I'm reading the same rule as you, but I read it differently, Kiirnodel. Especially since, out of everything, there are only a few 'hard stops' in place: reviving the dead, time travel, teleportation, etc.. Everything else is either possible, or may be possible, if someone figures out how.
You are reading 'only X, Y, Z spells should be permanent' as 'only X, Y, Z spells
can be permanent', which is not what the language indicates. It is a balance recommendation thing, and I've seen similar language in other games before. It is a warning that going outside of X, Y, Z for permanent spells can cause things to be unbalanced, which is why it states that GM should make the call on whether it is allowed. Compare that language to what is used in the 'Limits of Sorcery' sidebar in that same section, which is distinctly full of language like CANNOT, with not even a hint of being able to ask for GM approval.
When you have things that are explicitly disallowed, that implies that everything else is possible. But as Ian Malcolm would say, you can get too focused on whether you could do something to think on whether you should, which is why you have the language we're both referencing.
So, by everything I've seen, it is not against the rules to make a permanent petrify spell, but it can unbalance a game quickly, just like something such as superspeed can screw with a game in a superhero system. So you need to be aware of the pitfalls before they happen. It is like on old maps where people wrote "Here be Dragons". Doesn't mean you CANNOT go there, but, well, probably not a great idea unless you really know what you're doing.