It is not that they do not allow other render engines. It is that APIs to dynamically execute native code are locked down on platforms such as Windows Mobile and iOS. If you cannot dynamically execute code you cannot build a JIT JS engine which makes your webbrowser too slow compared to the competition.
Tribes 2 had a full web browser in the game. It was a continuation of having a full IRC client in the first game, Starsiege: Tribes.
(Tribes 2 also had the IRC client. The IRC client initially drove players into chat channels to help them find ongoing games, and dropped a link when someone left IRC to join a game. It was decent matchmaking in 1998, and helped build the community as well.)
The browser itself wasn't as insane as writing the entire game menu in OpenGL, which at the time was uncommon. Game devs cited this choice (instead of using a simpler menu) as causing a 6-month delay in the project, leading to much less testing and a game that crashed frequently for the first few months after release. The ubiquitous "Unhanded Exception" pop-up became a meme for years after.
Pretty sure they don't allow you to download and execute code even if not using a JIT, unless it is an educational app. There are multiple policy reasons browser engines can't be on iOS.
