It really depends on which mice were being included with the PC. The vast majority of PCs that shipped with mice shipped with symmetrical mice for this very reason. We're talking dating back to at least the 1980s. By default, the buttons were assigned so that the "main" click was the left button, so that obviously was meant for right-handed users, but even in the days of Windows 3.1 (and probably even earlier than that), most GUIs had the option to swap the buttons for left-handed people.
The whole "catering to left-handed people" schtick was Apple's own marketing around including only a single-button mouse with their computers, even after Mac OS itself gained contextual menus.
But yes, including a right-hand only mouse with a PC is not ideal. Most PC manufacturers didn't do this (many clone builders, on the other hand, often included the MS Intellimouse, probably because it was popular and MS was selling them to OEMs for very cheap - that meant lefties had to shell out for a different mouse).