Depending on the size of the head, and other characteristics such as scalp laxity, and the type of procdeure (strip vs. FUE) some donor hair can generally be taken from the sides above the ears. However, taking donor from the sides has severe limitations, and if too much is taken, this can cause several problems:
1) As grafts are removed from this region, it will "thin out" noticeably a lot quicker than the thicker donor area in the back. This will also be worsened by the Norwood degree of hairloss. Generally, the higher on the Norwood scale, the less side hair can be taken because a great deal of it may already be gone/miniaturized from male pattern baldness.
2) The scalp on the sides may become too tight (if strip hair transplant is done) when the donor strip is stitched/stapled back up after the strip is harvested. Over time, This may cause the pre-existing hair in the area to "stand up" (or start growing in other odd directions) rather than lay down. Aeshetically, this can wreak havoc and become unsightly and require "repair" hair transplant work to correct. To correct this, usually more grafts have to be placed back into these regions. So then, why remove it in the first place?
So this can be done, but hair transplant surgeons shun away from this in certain situations if they see potential issues that it may cause a patient. They tend to take a "conservative" stance on this, in other words.
