Overbooking is not the only reason that airlines have to deny boarding. Sometimes, they end up making an aircraft change. Sometimes, headwinds dictate that they have to lower their weight. Sometimes (as in the now-infamous United case), they decide that they need those seats to carry crew. Sometimes, a seat is broken, and they can't use it.
JetBlue is an airline that does not oversell seats (Southwest apparently also decided to join them in that practice this year -- my guess is that their unique no-assigned-seats boarding method gives them a lot more flexibility). Despite that, in 2016, they had one of the highest rates in the industry of forcibly denied boarding, significantly above the industry average. (source: https://www.fool.com/investing/2017/04/16/1-us-airline-never...). Each and every one of those had one of those "true reservation, it's not a standby ticket" tickets that you're talking about.
Practically, when picking who to deny boarding to, airlines have some pretty clear rules. If you want to be sure you're going to get a seat, pick one for yourself when you make your reservation, check in as early as possible, and buy a refundable ticket.
But that is itself a problem: that they're banking on you not knowing your rights because the exact terms are so complicated.
It would be a different matter if each purchase made it explicit:
"This is really just a standby ticket with an X% chance of having to bump you, in which case you are paid $Y and booked on the next flight. To convert to a true reservation you may pay $Z."