In the world of laptops, cutting unnecessary corners is the norm. Don't expect logical design choices. They'll do anything to save (literally) a penny.
Most laptops with optical drives run them on a SATA2 port, so it's not farfetched to expect the same for your secondary hard drive bay. If you had a HDD in that bay, you wouldn't have even noticed.
BTW, is there a dedicated 2nd hard drive bay in this machine (2HDD + Optical), or did you put a hard drive caddy in the optical bay?
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod