Yes, Google Play often lists applications as incompatible for some reason, likely because they are protected and only available to devices officially supported by Google Play. In consequence, they do not show up in the Market or Google Play app. (You can visit them on the Google Play website and see whether they are compatible with your device.)
Some people play tricks with their /system/build.prop file to bluff the Market / Google Play app into thinking it's running on a supported device. I haven't tried that yet on the PRS-T1 and don't recommend it.
The alternative is to find an alternate source for the application. Some developers distribute official APK packages from their own websites; or you could try an alternative market; some are mentioned in
this blog article. I found that 1mobile.com has direct APK downloads for many free apps, but I have no idea how trustworthy they are.