Because of the way heals operate. In FFXIV, a cure for 600hp to a person with 100% hp will still cure for 600hp and the healer will gain enmity equal to 600hp. I see this becoming a big problem later down the road... but for the time being we're ok.
On this note, Adloquium puts up a shield equal to how much it heals (double for crit) so 600hp to a person with 100% hp will still cure for 600 hp and will receive a 600hp shield (1200hp for crit) on top of their existing hp. In the event this individual has 3000hp and takes a hit for 3500dmg, he will stay alive with 100hp remaining.
