I believe that this happens in SQL 2000 because the compile "reads-ahead" or anticipates the datatype expected by the table column and then back applies it to the source expressions. Implementing something like this is highly dependent on the internals of the compiler which was completely rewritten in 2005.

So my guess is that that obscure feature was dropped as part of the rewrite. (These features can have some problematic side-effects too, though I cannot remember them at the moment).