They're just trying to make more money. You can use any shampoo you want. The thing to avoid is sodium in the products. That's why they say stay out of the ocean, pools and be careful with sweating.
My hair is fine and if it's not clean, it's limp and flat. Their shampoos don't clean enough for me. If you read the labels, most Suave shampoos don't have sodium. I picked up Finesse from Big Lots for $3. There are lots of others. Even if you use a shampoo with sodium, it won't wear off immediately, just a little faster.