Yeah, I understand your feelings on this. The Mac, much as I love it, doesn’t have a left facing pointer either. But it’s more obvious with the gloved hand. And so the right pointing finger and the mouse pointer tilted to the right remain. I’ve thought it’d be possible to recompile app_server (if I recall correctly, the cursors are in there from the last time I played with Haiku) to address this, maybe with something like a switched_app_server and app_server package that could have different cursors and be loaded into the package system via a Preferences pane (and a reboot).
However, in getting to the point, as for changing the position of the mouse buttons, open the Mouse preflet from Preferences. This works in both the BeOS and in Haiku. In there, you’ll see an illustration of a mouse. Click the numbered buttons to switch them. So, as @humdinger already had mentioned, you can switch 2 to 1 and close the window. It’s also possible to turn off the 3-button setting altogether and use a 1-button mouse like a classic Macintosh from the menu right above the mouse, but in doing so, you obviously lose middle and right-click.
(In summary, though, I’ll also add this: even if the feature to ‘flip buttons’ was in Haiku, like it is in Mac and Windows, I grew frustrated with switching the buttons around and just use the default primary mouse button; everyone else’s computer has ‘normal’ mice anyway).