The agreement provided traditional diplomatic immunity[1] to personnel stationed at the base and their families but only waived criminal immunity for personnel for acts outside of the course of their duties. This waiver didn't extend to their families, because not being personnel, all of their acts would be outside the course of their duties and it would render their immunity meaningless.
This year, they changed that immunity so it no longer applies to family members, meaning they no longer have immunity. It was not made retroactive because the US would not agree to that (ex post facto criminalization is allowed in the UK).
[1] Diplomats usually refer to this type of immunity as "special immunity" because it is not related to the performance of diplomatic functions.
The US has acted dishonourably in this case.