Difficult to enforce externally I think, except when an employee of a company shares an infringement with the marketplace or the creator

My assumption: the reason serious companies will pay is that they don't want to make silly mistakes like license infringement for which they can be sued. If they want to use your lib/tool/etc, because it will speed up their development or help they make money in any other way, they will choose to pay you these $100 per year. If you gain a few (dozens, hundreds, thousands) clients like this, you're able to build a solid source of income, and since they buy a license on a subscription basis, it's likely you will keep the clients for the next year