This actually got me thinking and I looked up the actual wording of that code... It says it is there to protect people other than the surfers that they may hit. From all that I've heard, surfers get hurt when struck by kayaks.
They'd have to rewrite this if anybody challenged it, also the wording along with the inclusion of kayaking specifically states that kayaking is not a legitimate sport...
17.12.510 Surfriders’ Beach--Swimming prohibited when.
The board of supervisors finds that the regulations imposed by this
Part 3, as well as safety requirements, make the use of surfboards impossible
during those times when the beaches are occupied by many swimmers and bathers.
The board of supervisors also finds that the use of surfboards and paddleboards
is a legitimate sport, and that there should be at least one area in which
surfboards safely may be used even during those days and those hours when the
beaches are most heavily attended, and that it is necessary, therefore, to
prohibit in such areas other persons so that such other persons will not be
injured. A person shall not swim, bathe or immerse himself in the waters of the
Pacific Ocean opposite Surfriders’ Beach or opposite any other area
designated, except while using a surfboard or paddleboard, or as necessary in
order to use a surfboard or paddleboard. (Ord. 9767 Art. 3 § 62,
1969.)
http://search.municode.com/html/16274/_data/TITLE17/Chapter_17_12_BEACHES_9_.html#54From firsthand experience though, some surfers are VERY territorial and an angry lot.