Consent is agreement to a transaction. A transaction that is forced upon a person without his consent is a form of aggression; it involves an unlawful (in a natural law sense) taking of life, liberty, or property.
It has been argued, consent must be fully informed consent in order to be legitimate. This is not always true. A person can consent to waive the right to fully informed consent. Suppose that I short sell you 1,000 shares of Facebook, Inc. for the price of $20/share on 1 January of next year. There is an important unknown variable here, namely the price I will have to pay to obtain that stock. If the share price rises to, say, $1,000/share before I have a chance to buy it, then I will incur substantial losses. There is no way of knowing for sure how much I will lose or gain, but if I agree to the transaction anyway, I have waived the right to fully informed consent. It was my responsibility to learn what it means to sell a stock short, and the risks involved, before signing the paperwork.
Many consensual transactions involve some element of the unknown. You agree to watch a movie without knowing exactly what the plot is or how much you will enjoy it. You agree to buy a lottery ticket without knowing whether it is a winning ticket. You go to the ballpark without knowing whether you'll get hit in the head by a foul ball. The list goes on. You realize there is risk, but you take the risk anyway.
Perhaps in some cases, there could be ways by which the person could learn more and thereby exercise more fully informed consent. For example, the purchaser of a lottery ticket could first research to find out the prizes and chances of winning. Or he could consent to forgo even the opportunity to forgo what limited information was available, and just buy the ticket without knowing the particulars. It happens all the time, and these people are not deemed to have been exploited. If they suffer, they are deemed unlucky and/or the victims of their own carelessness.
This may sound cold, but there is an important reason for this principle. A law to "protect" people from their own mistakes, e.g. through consumer protection or occupational safety laws, limits their freedom to voluntarily engage in transactions of their choosing. Such restrictions on liberty tend to have costs.