If I'm not mistaken, wasn't that also the case in 3e? The only time it wasn't the case, to my recollection, was in Neverwinter Nights.
The only difference, however, between the enchantment of magic weapons in 3e and 3.5 is that in 3e, a +5 Bow's arrows couldn't overcome damage reduction unless they themselves were enchanted as well- so a monster with DR of 5/+1 couldn't be beaten without +1 or better ammunition. In 3.5, however, an enchanted weapon bestows everything upon its ammunition, including its enhancement bonus for purposes of overcoming damage reduction. Another difference, though, is that in 3e, enhancement bonuses from a weapon and its ammunition stacked- so a +5 bow firing +5 arrows got a +10 to attack and damage. In 3.5, this is no longer so. Therefore, there's no reason to get ammunition higher than a +1.