Wrong. I've signed many support contracts that were negotiated basically to "one flat fee for everything we sell that you use, including future purchases[1]" for some period of time. Some people think that they 'optimize' contracts by pushing for "I just want a service contract only on the SKUs I have". That's not always a real optimization.
[1] Often, this includes a tacit "including things you are using that you didn't tell us about".
I'm wondering if this is something uniquely american, because we don't seem to have much negotiating power in the UK/SE.