I guess this is technically by design, but only if you have your iCloud account associate the number with iMessage (you toggle the send/receive from the number

I guess this is technically by design, but only if you have your iCloud account associate the number with iMessage (you toggle the send/receive from the number in the settings). So even if your phone isn't using imessage, if your computer or ipad is, they'll receive the imessages and apple doesn't also send a text in addition to the imessage for the non-Apple device.