All of the "smite" spells work that way. The language there is "The next time you hit a creature with a weapon attack before this spell ends, [stuff happens]." The range of the spell is "self."
It also eats your concentration slot, and has entirely different timing from the Smite class feature from which it is based. If anything, that's strong evidence for the designers simply not having a unified design mechanic.
Maybe they had enough time between those two books coming out for them to realize that the mechanics of the smite line of spells were insufficient. The PHB was a rush job with numerous such mechanical oversights, and it was too late for them to fix it, but they didn't want to propagate that flawed design with the Bladesinger. If Booming Blade took your concentration slot, then it wouldn't be terribly useful for its intended user.
So that wording is aimed at preventing giants from abusing green-flame blade and booming blade? Seriously?
No. There are no giants which know Booming Blade by default. Booming Blade was only ever
intended for use by Bladesingers, who were (apparently) limited to one-handed weapons.
What I'm saying is that, in the unlikely scenario that this spell ended up in the hands of someone it was never intended for, the DM would
figure it out.
If the target is not within the spell's 5-foot range, the spell fails. How much more precise does it need to be?
Are you honestly arguing that they
intended for this spell to work with polearms
if you take the Spell Sniper feat? You're arguing that the designers sat down and had a conversation, where-in they decided that Booming Blade
should work with greatswords and spears but
not with glaives or halberds (because that would be over-powered or not-thematic or something); but then they agreed that it would be perfectly fine (balanced and thematic)
if that glaive-wielder also had the feat that was designed to enhance precision sharpshooting with magic rays? That scenario seems
likely to you?