Here's a tip...
You might generate more useful response to your question if you pose it along with your own conclusions.

For example, instead of asking why B31.8 does not allow welding attachments directly to pipe wall where hoop stress exceeds 50% yield, you might want to want to add something like: Is it because B31.8 pipe, with simplified design rules to minimumize pipe wall for pressure, has little residual strength remaining for any additional localized stresses at attachments?