It is common sense, especially with a DM who loves readying. It's a head fake;he's throwing away one weak spell slot to avoid a Concentration check on his important spell. The downside is that he's inviting people to shoot him, which tends to be a bad idea when you only have a d4 hit die.
Note that this only works against enemies who lack Spellcraft. If it's bothering you that much, start sending him against other spellcasters, and have them ready to interrupt only significant spells. When he starts casting they just need a DC 16 Spellcraft check to identify the featherfall spell, and ignore it. Then when he begins the second spell, a fireball lands on his head.
Even without Spellcraft, NPCs have knowledge of the gameworld; they know that some spellcasters can throw more than one spell per round. Have most of the archers ready to interrupt the first spell, but a few stay ready to interrupt the second. (This should especially be done by any repeat enemies, who have seen him use the tactic before.)
Note that this only works against enemies who lack Spellcraft. If it's bothering you that much, start sending him against other spellcasters, and have them ready to interrupt only significant spells. When he starts casting they just need a DC 16 Spellcraft check to identify the featherfall spell, and ignore it. Then when he begins the second spell, a fireball lands on his head.
Even without Spellcraft, NPCs have knowledge of the gameworld; they know that some spellcasters can throw more than one spell per round. Have most of the archers ready to interrupt the first spell, but a few stay ready to interrupt the second. (This should especially be done by any repeat enemies, who have seen him use the tactic before.)