Because AT&T is so trustworthy in comparison?

In fact, AT&T is much less trustworthy: https://www.eff.org/who-has-your-back-2013

Not to mention that others snooping on open Wi-Fi is more likely to cause problems.

Well, the only way to avoid that appears to be to go off grid. Perhaps some coffee shops will begin to advertise lack of wifi, and perhaps even relocate to areas without any mobile phone coverage?

Coffee shops already do the no wifi thing. There's no wifi at Blue Bottle, 4 Barrel, Sightglass, etc.

and without customers?

Plenty in the cafes I go to

Just not the ones on HN.

UK:Note mobile coverage is generally very good in UK but there are surprising dead spots, especially in 60s vintage office buildings.

hipsters will surely find it /snark