Pretty straightforward: The Jizya was paid by non-muslims who, by virtue of not being muslim, were ineligible for military service and thus needed the 'protection' of the muslims. Therefore, in addition to the tax bonus Jizya provides, it will also reduce troop numbers. This will allow Muslim states to be more unstable (needed) and possibly even force them to rely more on mercs, resulting in the occasional merc company forming an independent state (quite historical).
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