Is there any regulation or law such that a Canadian citizen who travels to US to have and show a ticket at the border for returning back?
There is no regulation or law that makes it mandatory for a Canadian citizen (especially if you are entering under the Western Hemisphere Travel Initiative) to have a return ticket when entering the United States. However, as with any other government, the United States (and by statutory extension, CBP officers) can deny any person entry for any reason, including and particularly if a person seems to intend on staying past their permitted length of stay.
For that reason, if you are entering the United States by any means, even if by car, you should be fully prepared to answer 1) how long you're staying, 2) where you're staying, 3) when and how you're leaving the US, and 4) why you're entering the US.
Just as a side note, you can read about someone who was denied entry for a similar reason, but note that this individual also screwed up his answers when he was questioned by CBP. (and he did eventually get into the US).