Poor Julie, she just can't figure us Yanks out. She keeps expecting logic to have something to do with our usage conventions.
The American rule is that double quotation marks are the standard form, and single quotation marks (what the British call "inverted commas") are normally used only to enclose a quotation within a quotation.
The British usually, but not always, reverse this order, using single quotation marks first, and then double quotation marks to enclose quotations within quotations.
Henry Fowler (Fowler's Modern Usage) claimed that starting with double quotation marks "is clearly less reasonable" (591), but he did not expect the single then double sequence, which he preferred, to maintain its primacy:
. . . as quotation within quotation is much less common than the simple kind, and conspicuousness is desired [for the more common simple quotation], the heavy double mark is the favourite. (591)
At present, even many who follow British usage conventions for other forms of punctuation will use the double mark as their basic form for quotations. In fact, Julie wondered why I was enclosing nonquoted words in double quotation marks, when I was not after all quoting anything. The question itself implies that double marks are the preferred form for quotation.
And that brings us to the point about using double quotation marks for enclosing words or phrases that are not actual quotations but that are being set off for some other reason--i.e., words referred to as words.
When a word is being defined or otherwise set apart as a word, it needs to be rendered in some way that will distinguish it from the rest of the text. For words presented as vocabulary terms, either italics or boldface can be used.
But when a word or phrase is being used archly or in some specialized sense, then it is more common to set it off with quotation marks, and since in the U. S. the standard form for quotation marks is the double mark, that is the form we use to set off such words or phrases. (For an example, look again at the title of my article in the link above, or check out the way I refer to words throughout that article.)
When many words are going to be set off, a writer will sometimes choose italics rather than quotation marks, simply to avoid the cluttered look of a series of individual words enclosed in quotation marks. Otherwise, it is more common to use quotation marks for this purpose.
Many forms of punctuation have been dragged into service to perform functions different from those they were originally meant for. There is nothing wrong with using double quotation marks to set off words referred to as words, as long as the convention is widely recognized. In the United States, this convention is universally recognized, so it serves its purpose quite well.
However, even those British writers who follow the American convention of using the double quotation mark as the standard for simple quotation will usually revert to the British style of using single quotation marks when setting off words as words.
In fact, even some American writers follow the British convention of using single quotation marks for words that are not actual quotations but that are being set off for some other reason. And when American writers use the British style of single quotation marks for such words and phrases, they also usually follow the British convention for placing periods and commas (outside the quotation marks, as opposed to the American style of placing them inside):
Mark that spot with an 'X'.
Please explain exactly what you mean by 'conventions'.
Even in the U. S., some scholarly disciplines, notably linguistics, philosophy, and theology, also use single quotation marks to set off words.
In linguistics, a word that is being discussed is italicized, and its meaning follows, enclosed in single quotation marks:
Faith 'to believe in something in the absence of evidence' has the same root as vatic 'seer, prophet, visionary'.
In philosophy and theology, terms with specialized meanings for that discipline are also commonly enclosed in single quotation marks:
There is an essential difference between 'being' and 'becoming'.
In these disciplines, too, when single quotation marks are used to set off words, commas and periods are placed according to the British style, outside rather than inside the quotation marks.
Those who write in such specialized disciplines are given style sheets by whatever scholarly journal they plan to publish in, so that they know exactly what form to follow in these matters.
The rest of us need not trouble ourselves about such things. We should just stick with the conventions that are already familiar to us, so that we don't commit the crime of stylistic inconsistency, which is always a danger when you try to adopt someone else's way of doing things.