It's a bummer for consumers, but you're absolutely right. It's no different than exclusivity windows for music releases, movie releases, or console video games.
If I recall correctly, PvZ was originally an iPhone title, then became popular, other platforms, etc. Maybe Apple just wanted to also ensure that PvZ 2 was an iPhone only title for a while as well.
For anyone else interested: http://en.wikipedia.org/wiki/Plants_vs._Zombies