Most iOS app can be installed on iPad.Unless you have app requirements that prevent the app from running on iPad, you can't stop anyone from installing and running it on their iPad. They run at a weird size and generally look "wrong" but better than not running at all, usually.
I've heard but never confirmed that App Store reviewers use iPad to review iOS apps.
If your iOS app was rejected for not being compatible with iPad, it may be because
it crashes on iPad when tested
the layout is strange enough on iPad to make it unusable (missing important button, things overlapping to become unusual, etc?)
If you can test your app in the simulator, test it on an iPad simulator.