As of the latest news..they are negotiating for it..
It just happen that the US has the best weapons on the market and available to KSA.. so why not..
Read also about KSA buying the S-400 and other systems from Russia and China.. they buy from France a lot and Great Britain too.. and lately 5 new corvettes from Spain and another 5 from Turkey.. but bear in mind that the new Saudi policy in arms procurements involves ToT as a must in any deal even with the US..
That is because KSA has started making its weapons already and aims for 50% local production of its systems.. it concentrates on high tech and electronics.. and now it is going for lower tech systems too to be fully manufactured locally.. the Kalashnikov 103 is a good example..