Hi!
Maltego is a passive reconnaissance tool so there’s no need for permission.

You’ll get this Bing warning only when you want to use a transformer that relies on search engine, the reason for this is Bing started charging people for using their search API, so the only solution is to purchase a license, but you can still use all other transformers that do not rely on search engine.
The search engine transformers just use Bing anyway to search for result, so you can just do that manually yourself, the only disadvantage is that you’ll get the result in the web browser instead of having them in a nice graph.

Also you can ask for a maltego full version trial:
https://www.maltego.com/trial-signup/

Greetings!
Diego