CPMInteraction portals use a "Header" screen layout whereas User portals which show up the hamburger menu on Mobile devices will be using a screenLayout which has left/right in it., screenlayouts like "Header Left", Header Footer Left", "Header Left Right" etc...
This is an expected behavior when the harness of portals use screenlayouts which don't have right or left in them.
So it's like only the left and right panels of the screenlayouts are collapsible and they can hence have a hamburger menu on mobile devices. But with Interaction portal, it's not possible.
I think they don't have a Menu generic to all tabs, like a menu that should be available always irrespective of the tab they're currently in. That could be the reason they chose "Header" screen layout. I'm not sure, though.