[{quoted}](name=Kitten of Evil,realm=NA,application-id=3ErqAdtq,discussion-id=YucLXnJ7,comment-id=000a00010000,timestamp=2015-02-03T04:26:59.178+0000)
Why doesn't it make sense? Presumably there's a point to %-based damages capped against minions (oh, it's because executing minions would offer no counterplay to an enemy jungler). Where the same exact logic applies to a non-%-based move due to infinite stacks, why should the exact nature of the move matter?
Because it's flat damage.
Percentage-based damage always deals a flat amount of damage after doing a bit of math. 8% of 2500 is 200 (which just happens to be the cap on Vayne's Silver Bolts vs Monsters). The cap keeps the %-based part of the damage from going over a certain flat value.
However, that doesn't mean they can't reapply the damage. Vayne can proc Silver Bolts as many times in a row as she wishes, always dealing that 60+200 true damage to monsters with >2500 HP, and it's always going to be that much damage per Silver Bolts proc.
With Kalista's E, the damage is already flat damage, plus flat damage per extra stack. The only logical way to hard-cap it would be to actually cap the number of stacks her E can apply. Alternatively, you could make her Rend deal reduced damage to Monsters, which tends to be an easier way to balance things that are too strong in the jungle.