The decision was made a very long time ago in the EDGE codebase to, by default, switch all switches that have the same tag and special.
It's not vanilla behavior but it's been that way since 1998 so we will not change it so as not break any EDGE maps that rely on that behaviour.
However, there is also a SWITCH_SEPARATE special flag for DDFLINE for the exact purpose of not having all lines with the same tag switch when one is pressed.