PIC (16F627A) inverted inputs
This phenomenon is known as a pullup resistor. As seen on the wikipedia page, it is widely used in conjunction with a physical switch. When pressed, the switch "pulls" the input low, overcoming the resistor's high impedance.

Also, on some MCUs there is an option to invert the pin's function. For example, on the AVR XMega, you can configure it so that both input and output will be inverted. I am not sure whether this specific PIC supports such feature. You'll have to dig through the datasheet.