There are reasons you can oppose this ban, but this isn't a good one of those reasons.
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The US promises certain freedoms and protections, and TikTok is saying the US is unfairly denying them those freedoms and protections.
China doesn't promise those things to anyone, so there's no grounds to complain they're not being fair.
I mean, imagine this argument applied to other things. "They don't allow <insert liberty> in their home country, why are they suing the US for denying <insert liberty>?"