Because there are people who would press that button on non-spam calls as well. And then litigation hell will break loose.
Or, if you phoned a store to ask whether they stocked an item you want to buy, do you think the store would press the button?
At any rate, people already (often) pay to make phone calls. By dealing a number, you're implicitly agreeing to pay. Just think of it as a charge (often waived) for the time of the person answering it.