I understand your frustration and you reasoning makes sense but it may be more helpful for you if you try to solve your problem on this site rather that complain about what has happened to you.
This is information from the iTunes store FAQ web site ......
If you download a rented movie on your iPhone 4, iPad, iPod touch (4th generation), or Apple TV: It is not transferable to any other device or computer.
That is the "to the point" part of the FAQ. This is the web site where you can read more if you like.
http://support.apple.com/kb/ht1657
If all else fails you can contact the iTunes Store here.
http://www.apple.com/support/itunes/contact.html
Now to try to help you watch the video, try quitting the videos app and restarting the iPad. From your home screen, double tap the home button and the task bar will appear with all of your recent apps. Find the video app. Tap and hold down on the icon until it wiggles and then tap the minus sign in the left corner to close the app. Tap the screen above the task bar to return to the home screen.
Now restart the iPad. Hold down on the sleep button until the red slider appears and then slide to shut it off. Hold the sleep button until the Apple logo appears to start up again.
This may not work, but it's not going to hurt anything and it is worth a try.