Just to clarify, the SLP plugin is not the issue. As you referenced this is something Google is now requiring for any new domains. This is not something within the power of plugin authors to control. Google is not selling an overarching license to any developer to allow them to use the map api for multiple new site domains is the way I am reading it. Each new domain has to have its own API key. We suspect that they will i the future push for pay as you go, as they mention the free API key allows quite a few look-ups per day. Speak to Lance on Slack and he can provide you more detail. He is back in the office starting tomorrow.
thanks for bringing it to our attention.