Why TimeMachine defaults to case sensitive? I've just had some problems with a disk that was formatted with HFS+ case sensitive while every else by Apple was non case sensitive.

After a long time moving things away from my biggest disk, partitioning and formatting non-case-sensitive, I've found out that Time Machine does format "case sensitive"

Why Apple wants case-sensitive only for Time Machine??? What's the technical advantage? I've just invested a long time fixing sync problems with my disks because of different case sensitivity problems
Case-sensitive and -insensitive filesystems under OS X are a mess (to say the least), primarily because some well-known and big software companies require their software to be installed on an case-insensitive filesystem. Otherwise these applications wouldn't really work because accessing IMPORTANT-RESOURCE.jpg would never work with a file named important-resource.jpg on disk.

OTOH for TM backups this is not really important because you will never run an application directly from your TimeMachine drive. Using a case-sensitive filesystem in that case covers both possible options without negative impact.