In my locality I frequently see small stores advertising "fishing licence" and "hunting licence", but rarely, if ever, "licences". If I saw this usage only once I would attribute it to some person's casual attitude toward spelling, but I see it frequently enough to be persuaded that it is caused by a widespread willingness to accept the final "-ce" sound as indicating plurality.
Lest it be suggested that such usage negatively reflects upon the literacy of the user, I would point out that Bobbie Burns used "acquaintance" rather than "acquaintances" in the first line of Auld Lang Syne.

So, I'd like to know...
a) Have you seen other examples of this?
b) Why is it done?

I'd say they're taking license with the spelling.

And of course we spell it "license," but I guess that doesn't make any difference.

Interesting, Duncan. I often go to Wisconsin where there are a lot of little places that sell fishing licenses. However, you are correct that they say, "Fishing License" or "Hunting License," but not in the plural. I am not sure why it's done, though.

Perhaps paradigmatically akin to dice and thrice.

UK usage:

Licence is singular; licences is plural.

To license is the infinitive; he licenses is the third person singular.

I suspect the reluctance to use conventional plurals in words ending with "s" or its sound, is akin to the reluctance to use proper possessives in similar words (which seems to me to be more common in US English than in UK English).

Hence the use of constructions like "boss'" (meaning belonging to the boss) rather than the correct "boss's".