malaysiacadet:
It is FAA policy to use this style of using your callsign on the radio. The reason is to avoid callsigns being confused with headings or flight levels.
So, "XYZ 240" would be spoken as "XYZ Two Forty" to avoid any confusion with Heading Two Four Zero or Flight Level Two Four Zero.
So when you hear an American crew using this style on the radio, they are not being sloppy but actually doing what they have been taught.
Personally I think it is a good idea.