This is usually true, but in the past Moodle HQ or core developers also had done some integration works "in-house" and not relying on the vendors (e.g. deeper integration with TurnItIn by Dan, Mahara plugin that was originally planned in Moodle 2.0, etc.).
For the case of TurningPoint (another competing "clicker" product), the vendor provided the integration, see:
http://www.turningtechnologies.com/studentresponsesystems/lmscontentintegration/
For the case of eInstruction CPS, someone in 2009 requested CPS-Moodle integration on their forum, but that discussion thread was locked and no further words on when the integration work would be done.
Hope this helps. I do hope that someone who has the bandwidth could pick this project up and get it implemented: MDL-26358.
tbh - with the massive range of mobile devices that students have these days it would make more sense for someone to improve the use of Moodle on a Mobile device... hold on ...that sounds familiar.... http://iphone.moodle.com.au/
With the $$ you "save" on vista licence fees you could contract a Moodle Partner to improve/develop a solution that might suit .... Drop me a line if I can help
TP's product is just compatible, not integrated, teacher has much export and import work to do...
When everything is automated and technology becomes invisible: this is REAL integration!
Right now we do integrate CPS, TP's, HITTs, vPads and many other brands...
I think you should take a look at this:
http://www.educlick.com/eps/EduClick%20Grade%20eInstruction%20v1.pdf
Regards,
Rafa