They usually leave the printer to its job because it has to do that anyway. Even when a printer prints a properly sized 300 DPI image, it must dither to its native resolution which works differently than the resolution a digital image.

Most printers need to use a pattern of dots to produce a single image pixel since a pixel can take on one of at least 16 million colors (For JPEG, more if converted from RAW) while a printer dot can take on between 3 and 12 colors usually with the possibility of varying the dot-size on some models.

The printer therefore uses whatever resolution it is given to fill the print area. In your example, then that becomes 100 DPI on input (where the D here really means pixel) and will be converted to the printer's resolution which is often 2400 to 9600 DPI (where the D here really means dot) and may not even be the same on both axis. In other words, there is a transformation from an image as an array of pixels to one which is a potentially non-uniform array of dots.

While you could resize yourself to a higher resolution, it won't necessarily help since there will now be two resampling passes which may introduce additional artifacts. Sharpening is sometimes done to offset from softness introduced by enlarging.