When configuring Hammer, it seems best to not set the default SolidEntity class to: worldspawn. Reason being - when you go to create a brush entity, you cannot change the class(I do not believe one wants a 'worldspawn' everytime they attempt to create a brush entity). I don't, currently, know the technicalities that prevent this; but, it is very frustrating and could inhibit someone just starting to learn goldsource map-editing in Hammer(most importantly, working with brush entities).
There may be other entities classes that cause this, if set as default in the 'Game Configurations' tab under tools>options.
P.S. There isn't any other instance on the web, that I could find, mentioning this.