I don't know exactly WHY, I just know that you should not use it.

Here was a question, BTW, may be you can explain

message #1387014

It's not an issue of the phrase "GROUP_ID" per se...you CAN'T use an expression alias in a GROUP BY. Period. You could even try to call it "Fred", and say...

SELECT MyField as Fred, sum(amount) as totamount
from mytable
group by Fred

...and it will bomb like Bristol Palin's boyfriend's condom. You have to refer to the original column name (or expression)