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 most cases, LOIs are not legally binding although many could contain specific provisions that are binding, such as covenants to negotiate in good faith, no_shop or standstill provisions (exclusive rights to negotiate, often coupled with earnest money deposits), and Non_Disclosure Agreements (NDA).