Simple, wye's look like a Y and tee's look like a tee. You can't discharge waste into the branch inlet of a horizontal sanitary tee, IOW a San tee on its back with the inlet looking up. A San tee in that position is not flow directional. The picture you posted of the wye with the toilet flange 45 degree fitting glued in works just fine as would an wye and 1/8th bend.