I think that might be specific to your carrier, forcing these network to be automatically joined despite your preference. I'm not seeing the same behavior with Verizon, where I get the managed networks but auto-join happily stays off.

I'm guessing MVNOs like Comcast would rather force you on their wifi if they can because this way they avoid having to pay the underlying MNO for traffic. Which might explain why they would force enable auto-join.

I'd vote with my dollars and pick a different carrier that doesn't have such user hostile wifi policies.