There are actually a number of instances where I don't want the constrain crop option turned on. One example would be when I straighten a landscape image using the transform tools and the warping creates some areas in the upper corners of the sky without any image data. Rather than cropping that out, I want that empty space included so I can pop over to Photoshop and fill it with sky.
Afaik, the only workaround to get the checkbox turned on by default is to create a preset with the checkbox turned on and apply that to your images on import. That's been working fine for me for a few years now.
I'm with you on the crop box popping back to "as shot" at odd times. I haven't quite figured out exactly when it does it and when it doesn't, but this behavior is definitely new since LR 6/CC for me.
And for print photos--yes, people do still want to print--most print labs require "standard" crops, not a freehand/creative crop. There should be a way to quickly and easily switch between the two modes explicitly, and then have it remain in the last mode until you explicity switch again.
Could I ask for clarification? Is this thread about the Constrain to Crop checkbox in the Crop and Lens Corrections areas that make sure there aren't any outside-of-image pixels in the crop frame, or is this about locking the aspect ratio with the padlock icon in the crop area?
I am confused because the title says Constrain to Crop that would be about the checkbox; however, there are also references to As Shot which is the aspect ratio not the constrain to crop.