Why "ETH" in degrees awarded by ETH Zurich? Why are graduates of ETH Zürich awarded with titles that include the "ETH"?

This list of degrees conferred by ETH also states conclusively that "[t]he Bachelor and Master titles may also be used in the form “BSc ETH” / “BA ETH” and “MSc ETH” / “MA ETH” without mention of the subject".

However, it doesn't explain the rationale behind the "ETH" appendix.
Is it because they think they are superior, and everyone should immediately recognize this from their title?
This seems to be historical. ETH is a university. Switzerland also has institutes formerly called Höhere Technische Lehranstalten (HTL), which are now called Fachhochschule.

The preconditions to enter the ETH and a Fachhochschule differ significantly. Finishing a degree at ETH used to take significantly longer, and the curriculum at ETH involved far more advanced mathematics.

Formerly the titles attained at a Fachhochschule was Dipl. Ing., Dipl. Ing HTL or Dipl. Ing. FH and the title you got at ETH was Dipl. Ing. ETH. As you guessed correctly, Dipl. Ing. ETH was a clearly superior different and more extensive education. The title from a Fachhochschule was roughly equivalent with a BSc while the title from ETH was comparable to a MSc from a competitive international university. This changed only recently, with the introduction of Bachelor and Master degrees in Switzerland a few years after the year 2000. 

Nowadays graduates of a Fachhochschule usually get the title BSc, and graduates of ETH usually get MSc degrees, and have the option to cut their studies short and end them with a BSc degree, although last I heard it was still very unusual to do so.