Public WiFi is only a risk if your computer isn't set up correctly...
Or perhaps you're using a device where the WiFi component lacks DMA and similar mechanisms to access or twiddle, directly or indirectly, memory or other components; controlled by a well analyzed, open source driver, or maybe a sandboxed driver running under a microkernel. This is more likely, except it's almost certainly not open source and we're basically taking the vendor's word regarding the architecture and correctness of implementation.
Sometimes it seems like we (technological adepts) expect people to be proficient in mechanical engineering because they own a car.