Nothing prevents charging for open source software. In fact, if a license forbids charging for the software, it is not free/open-source, because it violates the freedom to distribute.
Charging for open source software may seem pointless, because once the author has sold it to someone, they can't prevent the buyer from making as many copies as they like and giving them out for free. However there are cases where it makes sense.
Some companies make money by writing custom software, tailored for a client's very specific needs. The resulting software may be open source, either because the client requires it, or the company prefers it this way, or the software is derived from some generic software with a license that doesn't allow closed-source derivatives. In such cases, the customer pays because they want this software that's purpose-built for their needs, that nobody would bother to write if they didn't pay. Typically there wouldn't be many other people who'd be interested in having a copy anyway, and even if there are, the customer could be paying just to be the first to have the software.
Some companies charge for their packaging of the software. People may be prepared to pay to get nice installers, or even just binaries for their platform. If the software has a permissive license that allows non-free derivatives, the seller may put a restrictive license on the binaries that doesn't limits the number of copies the driver may make and forbids redistribution. If the software has a copyleft license, the buyer is allowed to redistribute the binary, so it's a small market.
In the days before cheap Internet bandwidth, most people got their open source software on CDs. The CD distributors obviously charge for their work of pressing CDs, shipping them, etc. The CD itself may or may not be copied for free, depending on whether it also contains non-free software and whether the collection may count as a legally protected database (see Distributing an operating system DVD bundling proprietary and GNU-GPL software?).
Paying to install an open source program on a platform such as iOS, where deploying an application requires a fee, is a more modern equivalent. Here the buyer is paying not because they can't obtain the software otherwise, but because they can't install the software otherwise.