I'm not sure that's correct, but it could be. It was my understanding it was a contract. That's why you have to agree to it - you are providing your consent and acknowledgement that you have read the terms and agree to abide by them.
If it was a license, they could simply grant it to you without requiring your consent. For example - I can buy a license for some shareware/trialware software by just sending them money and getting the license key in my email. Then when I go to use the software, it may ask for me to agree to the terms of service so that there is a formal agreement on the allowed uses of the software.
I'm not sure that's correct, but it could be. It was my understanding it was a contract. That's why you have to agree to it - you are providing your consent and acknowledgement that you have read the terms and agree to abide by them.
If it was a license, they could simply grant it to you without requiring your consent. For example - I can buy a license for some shareware/trialware software by just sending them money and getting the license key in my email. Then when I go to use the software, it may ask for me to agree to the terms of service so that there is a formal agreement on the allowed uses of the software.