The only way I can see this making sense is if Amazon could buy the supply chain relationships that HP must have built for their tablets. Probably HP have options/exclusivity deals on display and chip production for the coming years.
If we're referring to HP as a collective of things (i.e. individuals, such as the board) then it should be have ( - they have ....)
IMO I'd say it could be either really, though I would probably favour the singular.
I'm British if it helps...