Why do bard songs need a target?

[Archwizard]

If I had to guess, its to prevent pre-pull stuff, especially since the buffs proc without dots now.

Sure, but as OP suggested, they could easily go on the same system we see with abilities like Aetherflow that cannot be activated while out of combat anyway, preventing any sort of pre-pull.

As I was saying, being able to cast without a target would only affect downtime periods where enemies can't be targeted, and in that scenario (or even including pre-pull), without a target to uncap Bloodletter on, the most you would get is... pre-charging Army's Ethos, or maybe a single Pitch Perfect? Either of which is already a potential waste of buff uptime anyway...
Which frankly, the idea that a Bard doesn't get any sort of downtime effect, even just humming to pause their song timers, is already kinda curious.