I’ve had that happen but luckily not with anything like a mortgage, just smaller stuff that messed with my credit. I agree that it shouldn’t be legal - a loan/debt is a contract between the parties involved and if either wants to bring in other parties, they should have to renegotiate the contract (with possible exception of bearer bonds although those are mostly history).
I’ve told collection agencies before that I never made a deal with them and unless they can send proof that I did and can send the records, they should just have the original company get in touch with me and send updated invoice/records. They’ve never done either. I suspect because they usually don’t have any records and don’t know where they got my information from anyway.
Traditionally, it was a way to avoid/prevent takeovers. By taking public shares out of circulation, it increased the relative percentage held by insiders. But the last couple of decades have been weird. Tons of mergers, acquisitions, consolidation in multiple industries, yet the buybacks don’t seem related to that. Or worse, as if they’re designed for the opposite. Instead of preserving control of the company, they want it to get bought out, and the buyback is to net them a bigger share when it does. Laying off workers at the same time gives the company lower operating costs, making it more attractive for a buyout maybe?