The difference is M+ is not automatic que with an inherent time limit on how long it will take. Also has legitimate reasons for abandoning a run if the group can't do it. There is no way for them to tell automatically if the group chose to abandon the key, if the person didn't feel like being in a 2 hour run after 5 full wipes etc.
People certainly leave M+s for bs reasons but there are legitimate reasons to leave one. There is never a good reason to leave a solo shuffle. Absolutely terrible comparison.
Most of the time someone leaves a M+ for a bs reason they where also toxic and spewing garbage in the chat so report them for that and move on. Most of these type of people aren't silent leavers so there is a way for you to seek punishment for them most of the time.