Hi @ILikeHighEndGraphics - This is a legal restriction Google Maps places on their services. See Section 3.2.3 (e) of Google Maps Platform Terms Of Service | Google Cloud.
(e) No Use With Non-Google Maps. To avoid quality issues and/or brand confusion, Customer will not use the Google Maps Core Services with or near a non-Google Map in a Customer Application. For example, Customer will not (i) display or use Places content on a non-Google map, (ii) display Street View imagery and non-Google maps on the same screen, or (iii) link a Google Map to non-Google Maps content or a non-Google map.
If you have subscriptions to imagery from other data providers, you may choose to use those services in Cesium for Unreal that use common open standard for imagery.