423.003 likely did not apply there, because that's not really private property and I doubt the intent was to "conduct surveillance". (The term has a specific legal definition -- "Observation and collection of data to provide evidence for a purpose" -- and I'm not sure Texas has a more specific definition. Is looking for a cool picture "providing evidence"?)
Also note that APD's supposed ban says nothing of cameras, only of "drones". (No, contrary to what the media may tell us, R/C aircraft do not all have cameras or missiles.)
And of course "Reckless conduct" is vague enough that they could probably apply it to anything.
What probably did apply there is this NOTAM from the FAA which prohibits flying under 3000' over stadiums shortly before, during and shortly after events. State police don't normally enforce FAA regulations, but it's certainly possible.
That won't apply to the city of Austin during SXSW, and 423.003 probably won't apply to public spaces, but certainly, APD could try "reckless conduct" -- and even if the charges were eventually dropped because they don't really apply, that doesn't beat the ride downtown.