More relevantly—it's harder to make a marketing stunt out of a fallible mission.
And don't say launch services, 'cause you're using "it's a marketing stunt" to explain why they aren't taking payloads. "It's R&D" makes a heck of a lot more sense.
SpaceX is well known for making a marketing stunt out of their failures (see "How Not to Land an Orbital Rocket Booster"). In fact, Elon Musk is known to make marketing stunts out of anything.
But that's not it, they could have launched a dummy payload if they wanted to, they have already done it for other rockets, and yeah, they made it a marketing stunt. Famously, they launched Elon Musk's Tesla Roadster in the direction of Mars. But here, launching a payload at this stage of development is just not the best thing to do.