FYI the wiki could've answered your guys' question. Though, being all the "AoE vs. single target matters" effects are new for season three (Liandry's, Muramana, Furor enchant, Gunblade's new Reload passive), it's possibly just an oversight because Guinsoo says fuk permaslowing. (Frozen Mallet is, for some reason, totally okay.) Now why they coded it in the first place as AoE and made an exception for it to work fully with spell vamp, rather than the other way around (coded single-target with an exception for Rylai's), is beyond me...
And not that I play her but I like Tear->AA on Cass. Passive or not, and it doesn't actually help that much because it forces you to continually cast spells (i.e. spend mana) just to keep it stacked, a couple rotations of E will run her down pretty quickly. RoA's health is redundant with Rylai's, which she loves, without the resists to back it up. Grail is bad because she doesn't give a shit about CDR like other casters. All that leaves among major mana itemization is Tear, which she stacks very quickly and loves the ginormous AP it eventually will grant.
Muramana is not used legitimately on any AP casters. Casters, yes. :P AP casters specifically, no.