Where does it say that it should never be used as a preposition? "Round",when used as a preposition, is simply "around" with the "a" dropped. It has got nothing to do with the adjective "round".
I, and a few million others, use it that way all the time:
round (DIRECTION) MAINLY UK Show phonetics
preposition, adverb (US USUALLY around)
in a particular direction:
The garden is round the back (of the house).
I used to live round (= near) here when I was a child.
You must come round (to my house) sometime soon.
UK NOT STANDARD We're going round (= to) the pub for a quick drink.
http://dictionary.cambridge.org/define.asp?key=68764&dict=CALD
round (AROUND) MAINLY UK Show phonetics
preposition, adverb (US USUALLY around)
in a circular direction or position; around:
The Moon goes round the Earth.
We ran round (the outside of the house) to the back, looking for the dog.
The idea has been going round and round in my head all day (= I can't stop thinking about it).
When one engine stopped, we had to turn round (= turn to face the opposite direction) and fly home.
http://dictionary.cambridge.org/define.asp?key=68752&dict=CALD