My understanding, the Target is the object thrown not the object thrown at. As the object thrown need not be a piece of earth, casting requisites may be required. If I throw a rock, that's just Terram. If I throw a leg of lamb, that's an Animal casting requisite. If I throw a pie, that's Herbam (and maybe Animal)... etc.
Edit: Generally, I think we need better nomenclature differenting between Target (ex. object thrown) and target (ex. object thrown at).
Secondly, because the spell can be used to throw any solid object and, in the minds of prior ArmsM developers, that felt to be reason enough to assign it to Terram and make it generic rather than develop a whole bunch of Form-dependent throwing spells like Weilding the Leg of Lamb and Unseen Pie Thrower.
The invisible sling can be used to throw any kind of solid items, not just terram ones. It is your basic "throw stuff" telekinetic effect. You require requisites to throw stuff that are not directly covered by terram. So you can throw around a wooden plate, a log or a lamb leg, but you require the spell to have requisites to do that (He or An, in those cases).
Speaking about that topic: how much is it known, in the Order, to cast spells who use rego magic un-needing penetration, or, saying it another way, how many magi know how tu use the rego magic to harm people but with finess and aiming (and so, no penetration need)?
Or, in other words, agani, how much magi are things with MR killer?
The School of Vilano in House Flambeau is dedicated to fighting with magic that doesn't require penetration, either through the use of mundane weapons and magical defenses, or through the use of finesse-based offensive magic. And it's not like Flambeau are particularly secretive.
Here's a character concept: A vaillant flambeau, pure of heart, who struggles to keep his fiery, battle-lusting blood in reign while trying to be accepted by the order as the peaceful man he wanna be.
Wait... Sounds like white wolf to me l