Wow, I never noticed that either. That's quite bad.
Mind you, the only fire modes listed for weapons are SS/SA/BF/FA. Full-Auto fire is listed seperately and restricted to Full-Auto weapons, but that's the only complex fire mode that links itself to a 'simple' fire mode. That means that supressive fire isn't linked to any fire type (or weapon), but neither are S-A burst and Long Burst. By the exact same argument, you can do a Long Burst with a Single Shot weapon (which is obviously not intended).
The link is sort of hinted to in the text, but never actually stated (just as with Supressive Fire).
[Edit] Actually, SA Burst does specifically state it's an option for SA weapons: "Semi-Automatic weapons can fire a burst of three rounds with a Complex Action."
Long Burst hints to it like Suppressive does, but doesn't actually say it.
(Mind you, if you try to abuse that in an actual game, you should be booked).