Correct me if I'm wrong, but it seems like the original point here is not whether someone read the code and then contributed, but whether their contribution might (inadvertently or otherwise) contain Matlab intellectual property because they have been exposed to it. It seems a reasonable safeguard to prohibit those who have seen the source code from potentially contributing in a troublesome manner.
You're quite right about exposure to code: it's prudent to be hyper-cautious. But the original point of contention was the suggestion that it's legally required to be this cautious, that people simply don't have the right to read something and then write something similar which isn't a copy - and that is false.
