Given the number of people using iPhones, you basically have to support the platform in order to make any money, so I wouldn't say the agreement is really "voluntary".

The internal cert is different from a regular application cert. The conditions for use are explicit.

Apple and Google, these corporations signed a license agreement to conduct themselves a certain way and failed to do that WRT the enterprise org cert. They were not forced into signing an agreement and have access to excellent legal council. This is a manifestation of the prevailing culture.

Furthermore Facebook is ruthless about enforcing their IP to "their" data (also voluntarily offered by users) and Google dictates the same way, except much of Google's data is hoovered up. These companies have all have a history of dictating and exclusion.