Why do these two behave differently? I don't know, but T-SQL is providing you the two ways you can use as needed. In the case where you want all the data from the table deleted and the counter will restarting from 1, then truncate can help you. If you want to delete all the records but don't want to reset you counter, then delete is there for you.
Just thought I'd chip in with my explaination for this: With delete you aren't always removing all of the rows from the table so you wouldn't want it to reset the counter. Truncate does it the other way because you are always deleting all of the rows (and it gives you the option in contrast to delete).