For any platform where the same desktop may be accessed from different screen sizes, (ie, everything but Linux), we have to choose between resizing the desktop on each connection, scaling the remote display or adding scroll-bars. None of these are good options, but defaulting to one the latter two gives a better experience for users who want to change that default--we don't change their desktop resolution until they ask us to, and enabling resize-to-client takes effect immediately, whereas disabling it don't take effect until the next connection.

This was in fact the default in the early days of the product; I don't recall why we changed it.