Hey, don't worry. Most of this stuff is info you have to go looking for. Just use the cover that goes inside the seat on a bouncy chair or the like. There are other uses for the products. That's why they're allowed to sell them. And because the aftermarket products aren't federally regulated (in US and Canada), they can market them however they want.
The other reasons are also that car seat covers (that come with the seat) have flame retardant (and those covers don't), and the stiching is done with heavy duty thread. Have a peak at the seams on the car seat, they're pretty durable. And crash tested.
Welcome to parenthood, it's like going to school all over again.