What version of mojoPortal are you running?
If it's pre-2.7, you'll need to upgrade. Short story: mojo wasn't using the api key but 2.7 does and also provides a method (iframe embed) for not using the key.
Long story: when mojo originally implemented Google Maps, the api key was required. Google removed the api key requirement at some point and mojo no longer used it for connecting to the maps api. Google then decided they wanted to require it again but didn't make any breaking changes and mojoPortal wasn't updated to use the api key. Google decided to start charging for use of their service (sort of, you have to give them a credit card even if you're a small site that will never hit their quota) and they started placing the developer purposes banner on the maps when the key wasn't used. We re-implemented the key in mojo but before we made the 2.7 release we figured out how embed maps without using an api key, using an iframe embed. The api method provides a lot more functions but the iframe embed method is a lot easier and is sufficient for most sites. Stay tuned for when Google changes things again. They're good at that, it keeps them relevant (as if they need help with that, lol). ;-)