Here is another question for you. Why would anyone try and get you to sign anything unless it is binding and could be used against you in a court of law? There is no other reason.... this is the reason they try and get you to sign this. The only person that is being locked into anything is you and usually in a way that is very one sided and only benefits the carriers and the tower companies.
In conclusion, terms must be agreed between the parties before the contract is signed or executed. Even expressed wording in contract terms stating that the standard terms will prevail over others may not be effective where it can be shown that no agreement was reached, whether expressly or by way of conduct.