You can't even consider any other possibility? Interesting.
I'm not a Texas lawyer (California instead), but its law on trespass as reported by Texans on the Freep sounds similar to California law. The police can't make custodial arrests of people for trespass on private property in the absence of some form of authorization from the property owner. Posting a No Trespassing sign is generally sufficient, but often the property owners ask the police to act.
Nowhere has it been reported, that I've seen, that the property owners/store personnel asked the police to arrest trespassers or that no trespassing signs were posted (there wouldn't be those in parking lots - stores want customers to come in). Furthermore one of the news stories mentioned, and I saw in a TV clip, that the Houston PD chief wanted to know why the hell people were being arrested instead of cited.
All this together tells me that arresting ANYONE for trespass in these incidents is a per se violation of at least official police procedure, and most likely flat out illegal.
Since some of the people who were arrested had receipts showing that they were engaged in commerce with Sonic at the time of the arrest, then the cops did NOT see correctly. Maybe your supercops have ESP.
That's funny. I recall posting a picture on that thread of three women who were arrested at Sonic showing a receipt for food they had purchased at 12:35 a.m.