Why do we say "to be a laughing stock"?

I've come through the expression "to be a laughing stock" to talk about a person who has done something stupid and who people laugh at because of that, and I've started to wonder about it.

First of all, why do we say laughing using a gerund when this is normally used in active meaning, in the sense of someone/something which creates an effect (e.g. an interesting book, a cutting remark, a boring person, etc)? It would make more sense to say laughed, as the past participle is frequently used in passive mode, thus underlining that we are the object of other people's laughter.

Secondly, I've gone through the various meanings of the term "stock" (as in finance, animal raising, supply of things, food, punishment and other areas), but the only expression which seems to be partially relevant here is "stock" being used to indicate the degree to which someone is respected or liked by others, according to OED. However, the term in this meaning is indicated as uncountable.

Does anybody know what "stock" stands for and what the origin of the expression is? And let's not forget about the grammatical side to the question, please...

Stocks were a form of public humiliation used as a form of punishment a few hundred years ago.

To answer the first part of your question, laughing is the obvious choice. When you add the -ing suffix to form an adjective, you denote that the sentiment present in the word is transmitted to other people. An interesting book is a book that arouses interest in people. Therefore a laughing-stock makes people laugh at someone.

As for the second part of your question, according to Etymonline, this term was formed by analogy with whipping-stock. See the full entry here.

EDIT upon comment: I'm going to use the examples you give in your question.

An interesting book: a book that arouses the sentiment of interest to the readers.

A cutting remark: a remark that causes the feeling of pain to the receiver.

A boring person: a person that causes the feeling of boredom to other people.

All of the above adjectives could not have the meaning they carry with the -ed ending. Neither could laughed for the same reason. We have, however, interested readers who have the feeling of interest which a book arouses. We also have bored people who have the feeling of boredom that someone else causes (the analogous to cutting can't be used here as it is a different word). Someone who is a laughing-stock makes other people laugh at them. That's what the -ing ending shows.

I'll use the "something solid that things can be fixed to" definition of stock, which Merriam-Webster comes close to agreement with.

1 b archaic : a log or block of wood

In "laughing stock", laughs are the thing which are attached to the stock. "Laughing" in this context isn't the gerund; it's the present participle being used as a noun modifier.