Hello Anne,
The reason the Honor Code is an additional step is because WikiTree is created for a wide variety of users. Many of our members are simply family members who want access to the research others are doing for them. They aren't required to sign the Honor Code, because they are limited in what they are able to edit and access. In order to open up those possibilities, they are required to take that next step. Even so, even if they haven't signed the Code, they still have to abide by our Terms of Service, which still protects the integrity of the site and work of our members. We've carefully thought out the various member levels in order to accommodate everyone who has an interest in their family history, not just those who are skilled in research. We've also kept it as simple as possible, so as the site grows we aren't constantly making changes to the Code which would require people to re-sign it.
In regard to the spellings used, we are based in America, though we welcome and desire people worldwide to the use site. As such, though, American spellings are what are used out of consistency with our geographical basis.