Flashing side markers - bug or feature?

[davepl]

Put in new flashers today, went around to make sure everything works, and see that my front side marker lights flash with the signal lights (on that side only).

The also flash when the 4-way flashers (Hazard lights). They work whether the park/headlights are on or off (meaning even when on, they still blink off for the off state).

The rear side markers do nothing other than light up with park/headlights.

My 2010 Range Rover does that too, and even if not intended I'd like to keep it, but did they do that in 1970?

All my other vintage cars are 69 or earlier, and they definitely don't.