Especially when, in effect, what is being asked of them is to provide the LOI upfront to a little_known Internet_generated seller about whom they lack any familiarity with or whose bona fides as sellers they know next to nothing about _ other than, perhaps, that they (the buyers) had had some initial communication with the "seller" via an Internet contact.
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.