I agree. On the back generally advised. But consider you’ll never be able to rotate them safely unless you buy 2 more, as the fronts on a fwd will wear out faster and the rears will always be better. That’s why you should try to start with 4 equally matched tires. Caddyman is right about the differences in traction depending upon condition, making if a safety crap shoot when cornering that’s weather depended with just two tires being replaced on either axle.
To me, it’s how different the performance of the tires are instead of just, one set is new and one is not. But a tire company has to say something and at least to sell you two if you can’t afford 4. When will they say…“you’re safer driving with the older tires until you can afford 4” and run the risk of you walking ?