Is this a US thing?
No, these mistakes are done in a lot of places where training is done.
Wordwide phrase, like allready mentioned, it ready for departure.
the RT is very slack over there.
I hear the same stories and not just about R/T.
Anyway a short history: this phrase rule is implemented after the Tenerife crash with the two jumbo's where poor R/T was one of the problems.
That is why one should only use take off in a clearance, like Nashers said.