They use a flat iron because straight hair can frizz also. Come on now, let's not be unsympathetic just because they don't have curly hair
The idea that women with straight hair don't need to flat iron is as silly as the idea that we curly girls can just roll out of bed and "go" without any additional effort.

My sister has straight, thick hair and she flat irons her hair all the time. I'm pretty sure its because she wants her hair to have body and not frizz. I've seen her hair after she flat irons, and when she just brushes it straight, and I have to confess, it looks better when she flat irons. And she doesn't have any curl, or wave, at all. Not a bit. But she does have the same hair texture as I do: fine hair, but alot of it, so it's not going lie straight with no frizz.
Flat ironing doesn't automatically equal stick-straight, lifeless, and no shape. I think it does help give some body and shape and for some people, it does what they want, just like any heat styling tool. The trick is to know how to use, and not abuse, it.

Anyway, I recently got 2 nice compliments. One at a clothing store: the girl behind the counter asked me if my hair was naturally curly and what I used. I told her I made my own gel and she was very impressed She said my curls were pretty.
Then a couple of days ago, I was at starbucks getting coffee and as I walked out, these two guys said to me, "I really like your hair!" as I was getting into my car. That was kind of random but nice. LOL
That's me with FSG as my styler!!

3b. fine & thick, coarse, dry, medium porosity. Sucks up product and loves ACVs! FSG is my HG!

Currently using:
Low Poo: TJ's Refresh No Poo: CoWash w/VO-5; Sonoma Soap Company sulfate-free poo
RO: Yes to Cuc; SSC Lavender Condish; Sally GVP LI: Donna Marie Miracle D&C
Style: FSG w/Aussie Instant Freeze & CIAB