... it's called marketing and business ...
Love it or Loath it, this is just marketing 101.
Apple need to give the perception that their platform is the only one worth using or in fact, the only one you need.
If an application mentions the product is available on another platform, that message is lost.
Reality check folks - Apple is as much about money and control as it is about innovation and design. They are NOT a smiley happy bunch of open source hippies.
It's their marketplace, it's their shop front - they call the shots - if you don't like it, pack up your wares and sell them in another marketplace. Apple have never pretended to be any different (unlike Google)
Nice? No, it's not nice, it's ruthless, but this is business and it's ALWAYS been this way, since time immemorial.
You and I may cry foul. Some of us may vow to shun Apple. Apple don't care, because 99.9% of punters will never hear about Apples tactics and if they did, they won't understand, or care, or will just shrug and say "that's business"
Remember - it's all about choice - nobody is holding a gun to anyone's head.
When you submit your Application to Apples store, you do so understanding there's a hefty barrier of T&C's to wade through and one of those terms is bound to say "we can change these T&C's without notice"