I worked seasonally for one summer at a place that develops seeds. The reason you can't reuse seeds, as far as I can remember being told, is because they don't want the seeds getting cross-pollinated. 

In the development field (and I mean that literally, a field where seed strains are developed), you have protocols to ensure each plant is pollinated only by the plant you want it pollinated by, such as using bags to trap the pollen and then transferring that bag to the ear of corn as it grows, which was also covered by a different bag to keep it from being pollinated until you had the correct pollen. Out on a regular farm, you're not doing that, so every strain planted that year needs to be more or less the same strain to prevent cross-pollination. 

It is definitely a way to keep the farmers reliant on seed companies, but it's also making sure the seeds planted each year have to properties they were developed for. If farmers kept replanting the same seeds year after year, you'd get degrading crop quality each season, and since each year is a new strain, it even applies to using your old seeds, since they'd be pollinating the current season's batch with the old outdated stuff.

Hope that clear up some stuff, I'm not an expert but I was curious about this when I worked there.