I'm well aware of how contract law works.

The law requires that knowingly enter a contract and understand its terms. If this contract had been written such that they didn't realize he could reallocate investments after the fact, then you might have a point.

The law does not protect you from making bad decisions and entering contracts you probably shouldn't, provided you understand the contracts themselves. That would mean a huge majority of contracts are invalid because people frequently and routinely enter contracts which are inadvisable. (For example, subprime loans.)

Not understanding a contract and not thinking through the implications of said contract are completely different things.