That's not correct. IAPs can of course have price drops. There is just no way to track them with apps like Appshopper.
There are some developers who see no shame in sending 5 obnoxious notifications to make you aware of a drop in price for IAP. Not naming names but Gameloft are certainly culprits. I'd guess many freemium games do the same, but I can't say for sure because I don't have any on my device. If you want general IAP price drops - and not just those involving applications on your device - then you may be out of luck.
You should get AppTicker Push.. you will be able to track IAP price drops when the next update comes out