Hi Nathan,
The difference is that robots.ashx is a physical file but csshandler.ashx is not, it is declared in Web.config in 2 places, first in the system.web.handlers section which is used in IIS 6 or IIS 7 Classic mode
<add verb="*" path="*csshandler.ashx" type="mojoPortal.Web.UI.CssHandler, mojoPortal.Web" />
and also further down it is declared in the system.webserver.handlers section used in IIS 7 Integrated mode
<add name="CssHandler" verb="*" path="*csshandler.ashx" type="mojoPortal.Web.UI.CssHandler, mojoPortal.Web" preCondition="integratedMode" />
It should work out of the box unless they have it configured somehow to prevent registering handlers this way.
Hope it helps,
Joe