In some really old browsers the restrictions were not as great as they are in more modern browsers because it wasn't realised just how big a security problem that these were. Once it was realised then patches were introduced to those browsers in order to block the holes that were being exploited. Of course this meant that things that may have worked before then became impossible to do.
Basically this second group of restrictions allow the owner of the browser to decide how they want their browser to work and to disallow scripts on individual web pages from overriding their choices.
This article written by Stephen Chapman, Felgall Pty Ltd.