With letters of intent, it is critical that you understand whether the agreed terms are immediately binding or if there will be no binding agreement until all terms are nailed down.

Why were the companies which won licences allowed to sell significant equity shortly after being given Letters of Intent (LoI) resulting in trading of spectrum, an illegal act?

There are three basic types of written agreements, depending on the language of the agreement: agreements to agree, letters of intent, and a fully defined, binding agreement.