Where does it say they don't ? Usually the use of the word "parents" means both. Does it not ? If one wanted to mean only one parent, one would say "a parent" or "one of the parents".
Ted's dad wasn't an American Citizen. Was his mom a US Citizen ?
According to the US Supreme Court, someone born in America doesn’t need ANY US citizen parents to be a natural born citizen. See their lengthy discussion in Wong Kim Ark on the meaning of NBC.
Outside the US, born to a US citizen...depends. Many would say that since English common law acknowledged children born to UK citizens abroad as NB subjects (since about 1300, or at a minimum since the 1600s), that would fall under the Founder’s intent.
Since the only rational seems to have been to prevent foreigners from coming to the US and becoming President, and since the term “natural born subject” had included those born abroad for many, many years prior to the Revolution, I’m not worried.
To be honest, I find it incredible that anyone could look at Hillary Clinton without concluding that NBC is one damn stupid requirement - but one that Ted Cruz meets.