Normally designers of electronic devices (and any other things) try to keep cost and number of parts as low as possible. Every curve in a casing has a purpose, either aestetically or for technical reasons.
This device has a switch built in which has no electrical connection and no mechanical function... A free switch for everyone :-) Take it, it makes no difference.
There is another model of the Thermoscan that has a switch in that position, with markings for "Oral or Rectal equivalent".
The display location was also done that way, because the unit has a cover over only one side (covering the probe and the display).
I have that cover too. It was one thought that they wanted to protect the LCD with that cover...