I think the main reason is low power == less heat. That little Sherwood S-7100A is officially rated at 18 WPC into 8 ohms. However, if you dig a little deeper into the owner's manual, you will see that the 18 WPC rating is from 40 - 20,000 Hz. Using the more standard 20 - 20,000 Hz range, it's rated at 14 WPC.
The S-7100A was proceeded by an S-7100 model that did have a top vented panel. However it was rated for 25 WPC. I did read one account, but have not verified it, that the S-7100A had heat sinks on the output transistors and the S-7100 didn't. If that is true, that would further explain why the slightly higher powered S-7100 required top ventilation and the S-7100A did not.
Comparably low powered Sansui models of the same era also lacked top vents. However, unlike the Sherwood S-7100A, the low powered Sansui models did not have walnut veneer cases. That's one of the features that distinguishes the Sherwood S-7100A from other low powered receivers of the day. Most other brands used wood grain vinyl (or simply black paint) on the cases of their lower powered, lower priced models, reserving the fancy wood cases for their more expensive, higher powered models. In same instances (Marantz, Harmon Kardon), wood cases were available as an option. However, those optional cases usually had top vents, as they were intended to also be compatible with higher powered models.
If you don't need much power, the Sherwood S-7100A is a great sounding little receiver with an outstanding FM tuner. Just beware the on/off switch is integrated into the volume knob and is a common failure point. Keep DeOxit D5, or any other solvent based contact cleaner, away from it. The solvents cause the plastics in the on/off switch to become brittle and fail prematurely. Most receivers this age have some scratchiness in the volume pot and the impulse is to blast it with DeOxit. That impulse should be avoided with the Sherwood. You can read
more details here.