That would be one for the lawyers to answer/prove, but to be honest, System restore discs have a lower resale value than retail copies, so I would imagine that it would be illegal. Retail copies are far more versatile than backups, and I believe if you go to the trouble of purchasing replacement discs from Apple they are marginally cheaper than retail installers. I guess you are getting iLife as well with backups, but since they will only run on a machine which somebody has already paid for, the price is lower. You might decide to sell a retail Tiger on once you get a retail Leopard too. Can't do it with your backup discs.
The analogy to what you describe would be paying for a BMW 3 series and then stealing an M5 engine to put in it. Except you wouldn't be depriving anybody else of a copy by downloading it.
Copyright law is all about commerce, consequently rights of the parties involved are pretty well covered by airtight legislation. Downloading the retail is stealing. If you don't care, then hit up your torrent client. Otherwise bite the bullet and get your wallet out.
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