So why is it that I can feed magic to attacking allies, but not to economy allies?
The game is set up that way since it's usually a misclick to feed magic to an economy ally, since magic can be used as gold anyway.
There's an edge case where it's not a misclick: you stolen plans'ed a Wealthy Patron from a Vespitole player, and now you want to feed magic to your Wealthy Patron so you can feed the gold to either Apothecary or Charlatan. It's extremely rare, but everything would be cleaner if they closed that loop by allowing you to feed magic to Apothecary and Charlatan.
Charlatan and Apothecary were both added after the base code for the game was set up, though. So that edge case didn't exist yet.
It's usually a misclick to feed an attacking ally magic too, granted, but the edge case for that one is much more realistic: You control a Serpent Altar and an Orphan gang. There's a variety of reasons you might want to avoid proccing a Serpent Altar, and each of them are small and narrow, but you can add them up and they mean something. Maybe you want to kill Herd of Rats without smacking a Lepers and killing your Orphan Gang. Maybe you want damage a rampart down to 1 pt, but you don't want to destroy it entirely because then it will get shuffled back in (and there are enough Embargos or Fool's Golds around that you shouldn't just sit on the magic.) Maybe you don't want to proc the last charge of Paper Vipers and allow it to get shuffled back in.
There's also the good old, maybe you're Xalia and you want to destroy a Palisade so you don't die to Coup de Grace but unless you draw a Militia you literally can't shoot at the Palisade.
As a separate note, you can feed magic to Supplicant, but only if he is damaged! You aren't allowed to risk Cutpurse procs infinitely
.