It needs to put a good amount of the HERE services out there for everyone. Now that HERE and the devices teams are separated, there's little reason for preferential treatment, especially if Microsoft is interested in keeping other OEMs around. Samsung and HTC get a full suite of services bundled when they ship an Android device, including a highly praised navigation experience. What kind of value proposition is Microsoft putting on the table for those companies when asking them to pay to use Windows Phone?
(Don't say discounts on patent licensing, that won't lead to meaningful products.)