In Canada (again by the letter of the law) they can sell or not to anyone they want. However, we are also allowed to rent our houses/apartments to anyone we want. I didn't know you couldn't in the states.
I believe it is because they list the requirements of approval rather then requirements of denial. So they couldn't say, "we will not sell to you because you have a child under 5" but I believe they can say "we will only sell to people who's children are over 5".
And on a personal note, I think they should try their hardest to insure puppy goes to a safe loving environment however you really can never be 100% sure. The person could just say they have no kids, and really have 3 or 4 under 5. Or they could have no kids but 10 nieces and nephews aged 2-4 that visit every day for 8 hours that they just forgot to tell you about. (Extreme examples, I know, but I hope the point was there

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Or in another example "person 1's" triplets who are 2 years old could be much more gentle and loving then "person 2's" 10 year old child.
And, I hope no one was offended. You all have me worried now