The laws are - in the US you can’t make, sell, offer for sale etc. anything that infringes a claim in an issued valid, un-expired US patent. There is no special case for replacement battery packs like there is for medical procedures.
One of their battery pack patents in the suit. US7999510B2, is very broad.
The first claim, below, only requires a housing, lithium ion chemistry and a particular range of current at 18 volts.
It would seem to apply to batteries compatible with other brands of tools. From google patents it seems that the examiner considered 155 previous patents before deciding this was novel and not obvious. Reading the judgement the issue at court was not the inventiveness but wether or not a Canadian company who brought the concept to them initially should be considered the inventor. It was ruled that the Canadian one did not reliably put out the needed current.
It expires in 2023.
- A battery pack for powering a hand held power tool, the battery pack comprising:
a housing connectable to and supportable by the hand held power tool; and
a plurality of battery cells supported by the housing, the battery cells being capable of producing an average discharge current greater than or equal to approximately 20 amps, the battery cells having a lithium-based chemistry, the battery cells having a combined nominal voltage of at least approximately 18 volts.