"Quite clear" is not an argument, it's an empty assertion.

IANAL, but I know the rough outlines of US contract and employment law as a frequent party to contracts, occasionally to disputes, and as a US person who has both contracted others and been a contract worker.

If you have a contract (which doesn't even have to be written), you're a contractor. Full stop. And, an agreement to provide payment in return for performing certain actions (like installing an app, leaving it running, answering questionnaires, maintaining confidentiality, etc) is a contract, even if it's a clickthrough agreement. Ergo, compensated research panelists are 'contractors' in the eyes of the law.