Q:

'agree' as a transitive verb

[OED:] 15. {trans.} = To arrange or settle (something requiring the consent of several parties); to come to an agreement on.
In modern use chiefly Brit. Apparently RARE in the 19th cent.

I already understand, and so ask NOT about, the above definition. Instead, why's this chiefly British use transitive, whereas North American use of this definition, requires a preposition such as these. What does the preposition truly mean, if British English can omit it?
Footnote: I encountered this on p 187, Urban Geography, 3 ed., by Michael Pacione:

At the local government level,
Agenda 21 of the 1991 Rio Earth Summit agreed a
number of proposals relating to waste management,
energy conservation, the integration of land-use and
transport planning, and the protection of natural habitats.

A:

In AmE, we don't use "these", we use on . And we don't use "agree" as a transitive verb.. We use it as an intransitive verb.  
The preposition is not needed in BrE because the transitive verb does take a direct object. (V+DO)

British: We agreed a time for the meeting.
We agreed the terms of the contract.
American: We agreed on a time for the meeting. We agreed on the terms of the contract. Later, he agreed to (accepted) the terms of the contract.

The AmE version does not use a direct object.  The prepositional phrases "on a time" , "on the terms" are adverbial. 
"We agreed." 
"We agreed on xxx."
"We agreed that xxx."  (xxx is true)
"We agreed to [do xxx]."
"We agreed in principle."

