Sheet protection being active prevents any new mathematical formulas from being
added anywhere in the sheet - even in cells that have been set to 'unprotected'

Scenario: 
I have a set of values in a spreadsheet and a couple of them took a long time
for me to work out or might be difficult to follow when I return to the
spreadsheet later - So I protect those couple of cells.
How: I select the whole spreadsheet and do Format/Cells/Unprotect so that no
cells are protected. Then I pick the couple of cells I want to protect and
highlight them and set them to 'protected' through Format/Cells.
Now setting the sheet to protected allows me to overwrite all cells except them
two important cells.
So far so good.
I can type and write over most cells in the spreadsheet still, however what I
cannot do is add a new formula through 'Insert/Object/Formula' (that menu option
appears greyed out)

Suggestion1:
Protecting a 'sheet' should prevent amendments to existing formulas but allow
new formulas to be added.
Suggestion2:
Protecting a 'sheet' should only prevent amendments to existing formulas that
are anchored to a cell which itself is 'protected'.