The reason it's like that is if you have a game time scale of 0, you can still set some objects to be moving at a normal rate. If it was relative to the game time scale, setting a time scale of 0 (commonly used for pausing) makes all object time scales useless. Also making this kind of change causes serious backwards compatibility problems. If we did it, cue the forums flooding with "my game is broken in the latest release, Construct sucks!".
Changing an object timescale is just like updating a variable, there shouldn't be any problem with setting it to timescale * objecttimescale every tick if that's what you are after.