OK, you are quite right, that there's sometimes the "who's to blame"-reflex, but in this case there have been real impressive warnings before the event concerning the security at this special point of the route in the tunnel. It is not a coincidence, that there was such a terrible incident, but it really was foreseeable.
In this thread there is a reference to the derwesten forum (
http://www.derwesten.de/kultur/musik-und-konzerte/loveparade/Loveparade-wird-zum-Tanz-auf-dem-Drahtseil-id3265037.html), where normal people before the event were shocked that the route to the site was going through the tunnel and that the abandoned station was much too small for the million of people expected to come.
So no, it is totally legit to investigate who's to blame. This should be a warning too for the planning of the love parade 2011 in Gelsenkirchen (which will be canceled anyway, I'm sure...)