Voluntary Tax

I received a letter asking for voluntary taxes for the city.

It says, "You may pay all or part or none. Payment of any or all of these will be used to directly support the services indicated."

Could someone explain the logic behind people like to pay tax voluntarily? Why don't they just increase the sales tax and make it mandatory? Isn't that more effective? Who would pay for it? Sounds like wasting postage and paper to me. I never like these 5 letters together; either "T-A-X-E-S" or "T-E-X-A-S", but you can't escape from them.

In my opinion, "Taxes" and "Voluntary" are two contradictory words. However, I have a conspiracy theory on why the city sends out these voluntary tax letters. I suspect that the city may try to take advantage of people who don't understand English. As we have lots of non-English speakers living in this town, they pay everything that comes with the "tax" word.