In fact they even sell insurance for this, and companies that want you to use their software can offer indemnity protection with the same effect.
"What if somebody sues me because my use of your software constitutes a violation of their intellectual property rights?" – "Don't worry, we will protect you. Since you pay so much money and are a valued customer of XYZcorp, we don't want you to worry about such things. You'll be covered by our umbrella policy."
This conversation certainly happens, (although it almost certainly wouldn't have happened between any of "Rails" customers and the Rails core team.)
"What if somebody sues me because my use of your software constitutes a violation of their intellectual property rights?" – "Don't worry, we will protect you. Since you pay so much money and are a valued customer of XYZcorp, we don't want you to worry about such things. You'll be covered by our umbrella policy."
This conversation certainly happens, (although it almost certainly wouldn't have happened between any of "Rails" customers and the Rails core team.)