This is a known issue with the Twenty Ten theme, arising from both the theme and core code. Twenty Ten has a number of accessibility issues – see Trac: http://core.trac.wordpress.org/ticket/14782
Some of these have been corrected in a child theme for Twenty Ten that you can get here: http://accessible.sprungmarker.de/2011/01/accessible-1-0/ The CSS and functions in the child theme cannot correct the menu issues though.
Unfortunately, the menu issues are still present in the new Twenty Eleven theme that is coming with WordPress 3.2.