They're paying people to collect signatures... not for the signatures themselves. This is specifically allowed in most (all?) states for organizations to be able to pay people a wage to spend the time collecting signatures for ballot initiatives. Most states require the person collecting signatures to be registered with the state if they are being paid however. So unless they're doing that and sponsoring and registering every person collecting these signatures with the State, they are likely still violating the law there. Volunteer unpaid circulators don't have to be registered, but if you're getting paid for it, you're probably required to be registered to an organization that's responsible for what you do.

Note: This is usually done via a standard hourly wage, or a price per sheet of good signatures. Many sigs (I've seen 1/3 to 1/2) are thrown out because the requirements are extremely strict for processing. A good signature or sheet of signatures can be completely invalidated by an error made by the person collecting it, or because it's on a page with the wrong county listed (even though every signature line also requires an address to verify). Simple errors unrelated to signatures get entire pages thrown out.

Source: Was trained and registered as a paid petition circulator with the State of Arizona earlier this year.