You're ignoring the other half of the Rifleman's armament. The twin Large Lasers are capable of exceeding the damage thresholds on most fighters, and the AC/5s add in two more chances to inflict "lawn dart" checks (or more with Flak ammo).
Having more weapons and insufficient heat sinks is a problem in 'Mech versus 'Mech combat, where you're firing your weapons every turn, but not for an AA unit. So what if it builds 18 heat and you can only dissipate 10? You've got another 3 turns to shed that heat before the ASF comes back for another pass. More guns instead of heat sinks gives you a better chance to do serious damage (potentially up to 26 in this case) in the brief moments when you DO have a shot, because if you don't pose a credible threat, that fighter is coming back for another pass, and another, and another.
Twin Large Lasers are not a threat. They're constantly firing in Long Range, even at a fighter flying directly overhead at Alt 5. If it's acting as protection for units around it, the LL can only reach 5 hexes away from its position and will, most likely, catch the Side arc of the fighter.
4 (Gunnery) + 4 (Long Range) + 2 (Side Arc) - 2 (Anti-Aircraft) = 8
And that's if you're not using rules for velocity. If Quirks are in play, then you would obviously be using the Strategic Operations book, which would tack on another +1 to +5 for the speed of the fighter overhead. If they're not and you're using Total Warfare only, then it's a 10 for the LL, 8 for the AC/5.
FYI - It doesn't take me 3 turns to send a fighter around for another pass. 1 (if its Safe Thrust is 7+) or 2 turns, max. Unless the fighter is under 3/5 speed, which is almost impossible unless you're purposely using the primitive Bomber.