There would have been a power imbalance at the point of signing. I can well imagine that the implications of that particular clause weren't apparent at the time.

As a society (more so here in the UK than in the US, I'll grant) we have laws governing what one party may demand of the other. They don't prevent a genuine meeting of the minds, because enforcement of a contract will only be an issue if at least one party doesn't follow through. But they do limit the ability of the company to impose sanctions beyond a point.

One limitation in the UK is that penalty clauses that are "private fines", like this one, must be based on the actual damage caused.

In this case, as in the non-compete case, I would say that if a company wants to continue to influence what someone does, they should continue to pay them.