Rehosting and serving that file is costing the streaming company electricity, bandwidth, cpu etc. Your agreement with them is that you can press pay an unlimited number of times in a month, but they didnt agree to serve that file to other people in other households. It's not quite the same as lending someone a disc, because in this case you agreed to terms of service that specifically address sharing.
Going by that logic, if I watch a movie twice or more, I should be charged more? What is the difference, “electricity” and “bandwidth” wise if I watch twice, or me and my coworker watch once?
But that's moving the goalpost. You said that sharing an account has a cost. I ask you, if I watch once and my wife watches another time, how is that different from me watching once and my coworker watching at another time?
But you can lend those people a Blu-Ray, which only deprives you of watching it while they have it.