From what I have experienced with other brands, SiriusXM trial periods are subsidized by some combination of the mfgr and SiriusXM.
IMHO Elon/Tesla have no good business reason to put a penny towards SiriusXM when they are already paying for and offering FREE-to-the-consumer TuneIn and Slacker in even base Tesla models. Related to that, I doubt Elon/Tesla feels that their including a XM trial would be enough to incent some significant number of additional people to buy the UHFS Option who were not going to. I suspect Tesla provides the SiriusXM hardware with UHFS as it is, only because it's the least expensive way to have another "me too" item which Tesla can check-off on a competitive option checklist the press or future buyers may consider (luxury vehicles simply need the option to be competitive from some POV.)
From a SiriusXM perspective, knowing Tesla is "giving away" somewhat competitive Slacker/TuneIn on every vehicle, and only a subset of the vehicles that are sold are XM-capable, I suspect SiriusXM feels they have a very low opportunity of "conversion" (meaning free trial to fee) if they were to give away their service for some period of time.
I, e.g. had SiriusXM in my former Lexus and MBZ which I sold and consolidated to my single MS which has UHFS, so I moved over my few months of remaining SiriusXM subscription when I took delivery. Unlike some people, I do very few long-range hauls where SiriusXM cross-country station connectivity may be of benefit, and I can live without Howard and other exclusive channels that some people cannot. As has been said, SiriusXM prices have kept going up even for minimal music-only plans, so one does begin to think about that when they have alternatives -- especially free ones in your Tesla. Additionally, where I had nearly perfect SiriusXM reception in my former Lexus and MBZ, I ran into drops in areas I frequent with my MS -- IMHO likely because of Tesla's internal positioning of the SirusXM antenna (perhaps good for Tesla, not optimum for SiriusXM and some of it potential customers). As such, I cancelled my SiriusXM contract when it's prepaid service was up several months ago.
So, IMHO, while I too expected to have a free SiriusXM trial period in my Tesla like I had with other luxury vehicles, I think it comes down to a business decision by Tesla and SiriusXM to simply not offer it. (BTW, as odd as I thought it was, I knew before I ordered my Tesla there was no free trial subscription.
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