The problem is that you need permission. Whether the approval is usually given is irrelevant, what matters is whether they have the option to refuse.
Think about it like this: What would be the problem with the employment contract saying that salary can be decided by the company at will if they usually paid a certain acceptable sum every month?
I tried to distinguish "shall issue" from "may issue". A "you must let us know [so we can possibly refuse]" policy and a "you must ask permission [that will issue if there is no conflict]" policy are indistinguishable. A "you must ask permission and we reserve the right to say no no matter what" policy is terrible, and I'd not work under such a policy.
But the point is that it's not a question of probabilities, but of whether they are contractually obligated to issue approval unless specific conditions are met, so an implicit "[that will issue if there is no conflict]" is not acceptable IMO. There have to be specific rules for when they may refuse approval, so you can ask an independent judge to make a decision if you think the employer is partial in its decision.
