Where's that mandated?
For the United Kingdom, the main legislation is the Wireless Telegraphy Acts 1949, 1967 (WT Acts 1949 and 1967) the Telecommunications Act 1984 (T Act 1984) and the Communications Act 2003.
For the USA matters are much clearer: visit
http://wireless.fcc.gov/commoperators/wncol.html for the details.
Wireless communications are, like airworthiness and such, the subject of international regulation. No matter which country you are working in, somewhere on the flight deck of your aircraft there will be a Radio Station Licence. This licence will contain cross reference to the regulations under which it was issued. Those regulations will contain the details of who may operate that radio station and the qualifications required or the conditions under which they may do so.
I know that LAEs do jump into aeroplanes, turn on the radios and make transmissions without a care in the world, but some of their operating procedures and terminology leaves a great deal to be desired. (Even some of the B2s who ought to know better.
)
Added to which, they are committing an offence. There's more to aircraft maintenance than just the airworthiness regulations.