There are some people that prefer the screen be consistent regardless of the orientation. The screen orientation can be locked by choosing an Apple® function or by choosing a TouchChat setting.
The iPad® prior to version 4.2 has a switch on the side to activate the lock. The smaller devices and iPad® 4.2 and higher use a soft switch access by double tapping the home button and then swiping right on the task bar. Tap on the circle arrow icon on the left to lock the orientation.
To use the Touchchat screen orientation lock option: