A US citizen can not register a .ca domain by just being a US citizen;
.ca domain name registrar is CIRA and the policies are created and enforced by this registry. According to their policies, with protecting Canada and its resources in mind, they define Who can register .ca domain
A US entity such as an individual or corporation can not register a .ca domain unless they meet the Canadian Presence requirement policy set by CIRA;
It could also be a good extension for the state of California, but .CA is a country code Top Level Domain name extension (ccTLD) reserved for our beautiful country of Canada.
Highlights from CIRA Canadian Presence Requirement Policy:
.CA domain names are for only the individuals, companies or organizations that meet Canadian Presence requirements.
1 Canadian citizen.
2 Permanent resident.
3 Legal representative.
8 Trade union.
9 Political party.
10 Educational institution.
11 a university or college
12 Library, Archive or Museum.
14 Her Majesty the Queen.
15 Indian band.
16 Aboriginal Peoples.
18 Trade-mark registered in Canada.
14 Official marks.