“Because the share contains a file named exactly "--delete", and since it gets sorted first, rsync thinks it to be an argument and deletes everything.”
Also because Unix made the mistake (edit: calling this a mistake may be unfair) of having the shell expand wildcards. If it had provided a library for doing that, the “rsync thinks it to be an argument” part wouldn’t happen.
Alternatively, a file system could sort hyphens last when reading directory entries, but I’m not sure that would be a good idea. It would make problems rarer, but that also might mean fewer users would know when and how to avoid them. It certainly wouldn’t help with other file systems.
Right, DOS expects wildcards to be handled by programs. Besides avoiding problems from the shell expanding wildcards in problematic ways, this also enables some neat semantics like some tools accepting wildcards in multiple locations either for recursive path mapping or for a purpose similar to regex capture groups. It also substantially reduces (but does not eliminate) the need to be careful when including literal wildcard characters in commands.
The problem is of course that wildcard handling ends up being inconsistent between tools or entirely missing, although in practice this became less common as various Win32 calls related to file operations had internal wildcard handling, so applications got a basic form "for free".
As with many things in computing it's hard to say that either approach is superior to the other. PowerShell carries on the DOS tradition of not expanding wildcards in the shell, but provides a more complete and standardized wildcard implementation as part of the API to improve consistency.
In DOS/Windows the shell does not expand as the sibling comment says (which puts some burden on the programmer - I had to use a Windows DLL in a perl script once to replicate Windows-style argument handling) but as most people know they also have paths with \ and options with /, and / is not a valid filename character.
This use of / for options goes way back to VMS apparently[0] and changing the path separator to avoid ambiguity does seem sensible. Of course, Windows has its own quirks like CON.
Also because Unix made the mistake (edit: calling this a mistake may be unfair) of having the shell expand wildcards. If it had provided a library for doing that, the “rsync thinks it to be an argument” part wouldn’t happen.
Alternatively, a file system could sort hyphens last when reading directory entries, but I’m not sure that would be a good idea. It would make problems rarer, but that also might mean fewer users would know when and how to avoid them. It certainly wouldn’t help with other file systems.