Why does voice dictation not always appear on the new iPad?

The new iPad supports voice dictation for voice input, but apparently only sometimes. I understand that some things should not support it, such as password inputs and URLs, for example. What dictates (excuse the pun) where this functionality is offered, and where it is not?

The following conditions must be met in order for dictation to work:

• You must be connected to the Internet for dictation to work.
• You shouldn't be using audio on your device (e.g. if you are on a FaceTime call, the button will not appear.)
• You must be focused in an text field which is formatted for normal text (more about this below.)

Developers (and even Web Developers) can choose which keyboard layout will be visible to the user, via the UIKeyboardType property. Here's an example of several different keyboard layouts and what they look like on an iPhone (without dictation).

The dictation button is available in the following keyboard types:

• Standard (UIKeyboardTypeASCIICapable)

But is not available in the following:

• Telephone (UIKeyboardTypePhonePad)
• URL (UIKeyboardTypeURL)
• Email (UIKeyboardTypeEmailAddress)
• Zipcode (UIKeyboardTypeNumberPad)