1. If she has entered U.S. legally (with proper I-94) and overstayed, she has a very good chance of being pardoned and get a legal status by marrying a citizen. 2. However, if she has entered the country ILLEGALLY you are correct.
I am not an immigration attorney, but I personally know who went through 1.
Just my 2 cents.

Just as an example, if a U.S. citizen meets and falls in love with an illegal alien, I happen to think they should be allowed to get married and continue their lives here in the U.S.. As a U.S. citizen, I think my country should allow me to do that. Well, they don't - not anymore. This country is so interested in "punishing" my betrothed for the "crime" of sneaking into the U.S. that she is no longer allowed to legalize her status based on a marriage to me. This means she either stays here as my illegal alien wife, or I expatriate myself to go live in her country with her. Not exactly the kind of choices I think my country should be giving me - and that's just ONE example.