Additions to old code and introductions of new code often cause errors in numerous programs, that's why your pc restarts when it gets an update.
Your OS(Windows at least) needing a restart is because of files being locked and unable to be written to, hence why there's a pause on shutdown and startup for Windows to perform the changes before these are locked. This was one of the bigger changes from Windows 7 to 10, less restarts. AFAIK with UNIX only changes to kernel actually require a restart, due to how it is loaded at boot. Its just easier to force a restart for other stuff, making sure all dependencies are started correctly. That's also true for a majority of Windows programs claiming to need a restart after installation.
Bugs are a natural part of all programming, but at the level Summertime Saga is monetarily a proper QA cycle should have taken care of all critical bugs. The more obscure or unique ones are forgivable, but not the ones affecting every player and stopping progression.