They have no regard for whether computers get infected. It's even worse when it's a site for supposedly "legitimate" software (or any download) allowing huge green [DOWNLOAD] buttons and deceptive ads. These links almost always lead to spyware/malware traps. We all know it. Many of us wonder about how many people it hurts. It's obvious to most of us to decipher what is/isn't a correct file. However, it isn't obvious to a general audience. And that's a very serious problem for anyone who actually cares about other people.
It's also a reason I leave adblocking off for general browsing. I like to see which sites are careless. Regarding Paint.NET itself, it's one of the closed-source programs I use sometimes while on Windows. Sure, it's a good step above MS Paint. (Maybe one of these days it'll have basic effects brushes too.) Yet any credit I would've given to their effort goes right out the window with a shameful site. But that's a calculation for revenue they're making: one they'll have to live with.