The landing page has a Q&A. This is the relevant part of the response to the question, "Why aren't all GEO satellite links encrypted?"

>Encryption imposes additional overhead to an already limited bandwidth, decryption hardware may exceed the power budget of remote, off-grid receivers, and satellite terminal vendors can charge additional license fees for enabling link-layer encryption. In addition, encryption makes it harder to troubleshoot network issues and can degrade the reliability of emergency services.

So, the only suggestion that there would be greater heat/energy if they did encryption by default is the part about decryption (receiver) hardware having limited power budgets in some cases. There's more than what I copy-and-pasted above, but the overall message is that lots of organizations haven't wanted to pay the direct costs of enabling encryption... although they should.

EDIT: Link to Q&A https://satcom.sysnet.ucsd.edu/#qanda