Because ethernet chipsets are power hungry. The chipset alone pulls more than a complete Express does.
Is it over 180 degs? If not it is fine. Hot to us is not hot to a linear regulator. The max temp for it is 257 degrees. If it gets this hot it will shut off and cool down and power up again. But that will not happen I ran the same setup with no heatsink for months with out it being turned off and never reach anywhere near a temp that was concern. In fact the one demoed at LSH did not have a heatsink on it. I tested with out it and then added it to give us a bigger margin of safety. I had another option but I decided to keep it as is so the conductor could be heated. one of the items on it is not avaliable in a -40c model so for our extreme cold friend it will keep them from having issues.
RJ