Also your statement is not right, the fact that the Surface frame gets is only on small part of the frame. Where the fans are. COMPLETELY normal, meaning that its blowing all the hot heath outside!
Well, the frame has a definitely a passive role in the dissipation of the heat. Metals are very good thermal conductors, and the heat you can feel on your finger is heat wich is not stuck inside the box.
Of course, the hotest point is behind the location of the processor, but the whole frame gets warmer.
If the SP3 was made of plastic, you wouldn't feel this amount heat, and probably be like "wow, it never gets warm, that so cool ! hashtag life hashtag I love Microsoft hashtag chocolate", but the life expectancy of the device would be low.
The same for the MacBook, as they are made of aluminum alloy.
By the way, the picture you posted is very interesting, as you can clearly see the 2 little belts going around the fan towards the processor and that are used for the liquid/condensation cooling of the latter.