Why are responsive images opt-in?

Created by: peiche

It seems strange to me that Bootstrap, being responsive and mobile-first, requires an additional class for any image to behave responsively. The docs state that you must manually disable responsiveness, but conversely, you must add a class to images to enable, not disable, responsive behavior.

Obviously your choices are your choices, and I'm not looking to reverse that. I would just like to know the reasoning behind this decision, and I'm unable to find a past issue that might address this.