Dogma
Sorry matey but there is no way that "No ATC speed restriction" can be given prior to departure, same as we can't give you further climb or direct routeing. We may be thinking "he can go a bit quicker" as you depart but you're still with the tower controller (located miles away) and until you're on radar frequency we can't take off the speed restriction.
If it was taken out of the SID, a wide turn, slow ground speed anything of that ilk would jeopardise the following or previous departure. I know it can be a pain but this order is given with due regard to the prevailing traffic conditions. Let's not forget that there are certain a/c types that when told "no speed" actually slow down. When you're working to strict tolerances on deps (LL and KK especially) this can't happen otherwise you won't get the prescribed five miles on departure separations.
Our priority is not your noise footprint, just making sure that you don't bang into another plane.
P7