This is valid behaviour and I suspect that you could not restrict it out-of-the-box unless you write a Filtering XSLT to handle your specific special scenario.
The obilus (or divison sign or the ÷ symbol) is a part of ISO-8859-1 character set which is default character set on any browser and you can directly use these symbols in your HTML code in addition to using their character set.
You will get this expected behaviour (as the one defined for divison sign in your question) for other character of this character set as well like -
© ¥ £ etc.
You can get the complete list of these character set HERE