It's an impedance mismatch. You're moving files between a system that supports multiple streams per file (data fork + resource fork) to a system that has no con

It's an impedance mismatch. You're moving files between a system that supports multiple streams per file (data fork + resource fork) to a system that has no concept of different streams (POSIX). The extra stream has to go somewhere, or you get data loss. It's worth noting that almost all modern file systems support multiple streams. NTFS has alternate data streams, Ext4 has xattrs. Modern SMB and NFSv4 also both support this at the protocol level. The problem arises when you're using Samba (without vfs_streams enabled), or you're writing to a legacy FAT filesystem, in which case you start getting the AppleDouble files - again, to prevent data loss.