I guess I could possibly understand, with the unlocked quads and HT quads, since the stock heatsink was basically underpowered for cooling those CPUs.
But the G3258 dual-core, shipping with the copper-cored heatsink, can cool it pretty well even overclocked to 4.0-4.2 or possibly more if you aren't juicing it with voltage.
I think that it would largely destroy the value proposition of the G3258 if they stopped shipping a cooler with it, and purchasers would have to drop $30 on a 212 EVO or something equivalent. Not to mention, some people put G3258 CPUs in SFF machines, where a tower heatsink wouldn't necessarily fit.