Just like SK already concluded: it's a disincentive for minting on multiple branches. Although the economical incentives of doing so have never been good (just a little bit of accumulated interest in the end...), they're defeated with this protocol change.

With the current protocol you have a small chance of raising your accumulated interest over the accumulated interest of minters that only mint on one branch. You can do that by minting multiple branches, having earlier success and receiving more acumulated interest by doing that.

With protocol 0.5 the odds are you lose accumulated interest when tyring to mint on multiple branches (once again compared to minters that only mint on one branch). Duplicate stake detection punishes minting on multiple branches by removing a block that was detected by duplicate stake detection. Successful minting is no longer accelerated by minting multiple branches but delayed (until minting on multiple branches has been stopped!).