>As far as I understand, a contract requires agreement from the contractor
Maybe you are are thinking what is called express contract where parties state the terms, either orally or in writing. Two other types of contracts exist. Implied-in-fact contracts, and implied in law contracts are also valid contracts. Contract implied in fact is inferred from the circumstances There is no need for expressing the intent as happens in express contract.
The language in GPL indicates contractual arrangement. This is indicated when referring to the effect of the "agreement" and to what conduct constitutes acceptance of the license.
for example GPL 1.0:
THE ACCOMPANYING PROGRAM IS PROVIDED UNDER
THE TERMS OF THIS COMMON PUBLIC LICENSE
("AGREEMENT"). ANY USE, REPRODUCTION OR
DISTRIBUTION OF THE PROGRAM CONSTITUTES
RECIPIENT'S ACCEPTANCE OF THIS AGREEMENT
Maybe you are are thinking what is called express contract where parties state the terms, either orally or in writing. Two other types of contracts exist. Implied-in-fact contracts, and implied in law contracts are also valid contracts. Contract implied in fact is inferred from the circumstances There is no need for expressing the intent as happens in express contract.
The language in GPL indicates contractual arrangement. This is indicated when referring to the effect of the "agreement" and to what conduct constitutes acceptance of the license.
for example GPL 1.0:
THE ACCOMPANYING PROGRAM IS PROVIDED UNDER THE TERMS OF THIS COMMON PUBLIC LICENSE ("AGREEMENT"). ANY USE, REPRODUCTION OR DISTRIBUTION OF THE PROGRAM CONSTITUTES RECIPIENT'S ACCEPTANCE OF THIS AGREEMENT