Note that "they" are one person, Colin Percival. That's all Tarsnap is; one person, some backup code he wrote, and data stored on Amazon S3. As the sole owner and only person working on a product aimed at a niche audience, I don't think it's unreasonable that he has a little fun with the way he runs it. If this kind of thing bothers you, this service probably isn't for you.
If prices were listed in dollars per GB instead of picodollars per byte, it would be harder to avoid the what-is-a-GB confusion (a GB is 10^9 bytes, but some people don't understand SI prefixes). Picodollars are perfectly clear — nobody is going to think that a picodollar is 2^(-40) dollars.
Specifying prices in picodollars reinforces the point that if you have very small backups, you can pay very small amounts. Unlike some people, I don't believe in rounding up to $0.01 — the Tarsnap accounting code keeps track of everything in attodollars and when it internally converts storage prices from picodollars per month to attodollars per day it rounds the prices down.
And finally, the price in dollars per GB are also prominently displayed, right after the price in picodollars per byte. So really, you're just being bothered that he's having a little bit of geeky fun even though it has absolutely no effect on you.
But also, you picked out only one of the three reasons he listed (http://www.tarsnap.com/picoUSD-why.html); the other two are also important:
If prices were listed in dollars per GB instead of picodollars per byte, it would be harder to avoid the what-is-a-GB confusion (a GB is 10^9 bytes, but some people don't understand SI prefixes). Picodollars are perfectly clear — nobody is going to think that a picodollar is 2^(-40) dollars.
Specifying prices in picodollars reinforces the point that if you have very small backups, you can pay very small amounts. Unlike some people, I don't believe in rounding up to $0.01 — the Tarsnap accounting code keeps track of everything in attodollars and when it internally converts storage prices from picodollars per month to attodollars per day it rounds the prices down.
And finally, the price in dollars per GB are also prominently displayed, right after the price in picodollars per byte. So really, you're just being bothered that he's having a little bit of geeky fun even though it has absolutely no effect on you.