More interesting to me is why so many of those people don't seem to care that they can't add change correctly.
I remember workbooks back in first or second grade, the ones with cartoon drawings of nickels and dimes. So many adults would fail those lessons.
Like so many things these days, it's an issue of lacking a basic understanding. In this case, the basic understanding of how values of money are expressed. There are two elements. One is the quantity, the other is the unit. How many? How many of what?
There is a difference between 50 US dollars, and 50 Lyra?
And there is a difference between 50 US dollars, and 50 US pennies, right?
So, if you see a sign that says: $19.50 you know what that means.
19 US dollars and 50 US cents.
or 19 1/2 US dollars, where half of a dollar is the same thing as 50 US pennies.
19 is 19 whole units 0.5 is a half of a unit, the unit in question is the US dollar.
When you see the cents symbol, "¢", the unit of measure has changed. You are now counting pennies, not dollars. Even drug dealers have to be careful to get this one right. There is a big difference between a kilogram and a milligram of a drug.
Thus, these two values are in fact not the same:
.50¢ and 50¢
The first one is one half of one penny. Not very much. The second one is 50 whole pennies.
Using a period does not indicate pennies, it indicates a fractional amount less then one.
$.50 does equal 50¢
$5 does equal 500¢
.99¢ is just plain wrong, unless you really do mean that you could buy one for a penny and some how get change.
I find that sales people standing behind a sign with lots of prices like .99¢ look like idiots, and even more so when they just don't want to know how to correct their signs.
It may seem like a silly nitpick, that anyone looking at prices can figure out what is actually meant, but that's not always true. When buying natural gas, water, or any other commodity that is measured regularly in small units, or fractions of a penny's worth, you have to be careful. This is true when paying for network bandwidth. There is a big difference between 5¢ a megabit and 0.5¢ a megabit. One costs ten times the other.
You could really screw up a recipe for brownies with bad math. Just imagine a half a tablespoon of salt instead of half a teaspoon of salt. bleh.