When you get right down to it, there's no real reason to buy any clothing or accessory like sunglasses that's specifically labeled for cycling. The litmus should be whether a specific product performs the desired function. If it does, buy it.

It's hard to get away from certain cycling specific items, such as bibs, standard cycling shorts, etc. But you don't have to wear jerseys when riding, don't have to buy a particular brand, etc. When you can save money, save it and use that money for things where the right product is more important.