Because both Axis and Allies in WW2 were not going to let morality or opinions get in the way of their advantages.

Hitler employed plenty of non-Germans in the military - just from the Soviet Union alone there were something like 800,000 Ostruppen, plus countless more 'off the books' as HiWis who were recruited on a more ad hoc level. The Waffen SS had non-Germans outnumbering Germans. There were Muslim SS formations and troops from India and plenty of other places that weren't The Allies employed Axis troops after the war to stamp out uprisings in their colonial holdings, so they weren't exactly fighting for 'freedom' either.

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The Nazis had plenty of horrible ideas on which races were superior and how they'd be arranged in the hierarchy of the post-war world, but until they won they were happy to let Slavic boys get shot for the cause. Especially as the war went onward and the manpower crisis increased the Germans became ever more reliant on non-German troops to maintain the regime.

Hitler had Slovakia as an ally because they were a useful puppet and were able to provide tens of thousands of soldiers to the war effort. Slovakia was not alone in this.