Is it acceptable (or even legal) to have "I agree to the terms and service" checkbox automatically checked when a new user signs up with our website? I'm trying to convince the person in charge to not have the developers implement that feature. What are the best arguments for leaving the checkbox unchecked on page load?
It is legal to do that, but you will gain no legal protection if the default state is to agree.
Unless a user explicitly agrees (which means actively doing something rather than not doing something), any legal agreement that you have will be unenforceable in court. This has been tried and tested legally, so it is one of the few areas that are crystal clear.