Literally not compelled in this case, the TSA signage says that the image capture is completely optional.
More generally, having your stuff screened for security to get on a commercial plane isn't a 4th amendment violation, the word "unreasonable" is right there in the amendment for a reason. You're in public in an enclosed flying object bringing your goods onto someone else's plane with 100+ strangers aboard, it is completely reasonable and necessary for the freedoms of everyone involved for the TSA to ensure that your stuff doesn't have dangerous objects aboard.
Don't forget that freedom also involves the freedom of other people to not be negatively impacted by you exercising your "freedom."