Motherboard does not shut off

I recently bought a new computer with a Tyan trinity s2390 motherboard.I have 256 mb of ram I am dual booting with Win98 and Win2000.Everything works great exept when i shutdown the computer the optical mouse and keyboard syay powered up.I have contacted tyan and they say this is normal on newer motherboard,They say it keeps the +5v powered all of the time.Has anyone ever heard of this and why would they make this an option of the motherboard.Also if anyone knows of a work around to shut these devices off when the computer is shut off i would lovew your advice