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The Transporter/Erector can only lift a complete first and second stage. That is not to say it requires the payload, but to say it requires the second stage.

Taking your question to next logical step, why do a static fire of the first stage engine with a $20 million dollar second stage on board?

Well, they need it so the T/E can grab it and hold the complete stack.

A better question might be, for a Static Fire, why were they fueling the second stage at all?

Having said all that, there is an additional reason to test fire with the second stage, fueled, and with the payload attached is that the vehicle behaves different fully configured than in pieces, and this is a static test of the entire vehicle.

From Reddit:

According to Peter B. de Selding on Twitter, SpaceX implemented an optional policy of attaching the payload to the vehicle earlier this year with some “insurer upset”, as it saves about a day during of launch preparations, and allows SpaceX to monitor the payload’s interaction with the vehicle. The customer can decline this option if they wish.