Note that making such a change could have noticeable consequences as far as performance goes. Where login based session authentication is used to cover access to everything in a directory, including auxiliary files such as static images included in a HTML page, you would incur the override of a commit on every single access, including to the images. In those situations it is preferable that only the page/code with represents the core resource should perform the commit for time accessed. The only way you can achieve this is to leave it as now and for the programmer to code the commit explicitly in the required place. If you auto commit on every access, you impose the performance hit on everyone and doubly so for existing code which already performs the required commit. Thus changing this may have a significant impact on existing code.
BTW, a lot of those other systems probably came along after mod_python implemented sessions. They also do what they do as part of a bigger picture of making it easier for dumb users to use the systems. An aim of mod_python on the over hand is performance, control and flexibility. By auto commiting, you have taken away just a little bit of that control as once done you can't revert the change. It is thus perferable that if you want auto commit, you implement it in a layer above the API that is provided. Since a good design would implement session creation in a web application in one spot, it means you only have to accomodate for it in one spot, so in practice it should not be a big drama for a user to do themselves.