It is a legitimate question and the frustration behind it is understandable, but the legal framework has reasons even if they feel disproportionate from the outside.
Voyeurism and secret recording of a sexual nature, which is what photographing people in private settings without consent typically falls under, is treated as a sex offense because it violates the same fundamental principle that underlies the more severe crimes on the registry. The victim did not consent to being sexualized, recorded, or viewed. Their privacy and bodily autonomy were violated without their knowledge or permission. Courts and legislatures have determined that this violation, even without physical contact, causes real and lasting harm to victims, including trauma, shame, and lasting psychological effects.
The broad registry categories reflect a legislative decision to group all sex offenses under one framework for monitoring purposes, even though the severity varies enormously. Whether that framework is well calibrated is a legitimate policy debate, and there are ongoing legal and advocacy efforts in several states to create tiered registry systems that distinguish between contact offenses and non-contact offenses. Some states have already moved in that direction.
On the parole question, the clarification is important. Federal inmates of any kind cannot be paroled. The federal parole system was abolished in 1987 for crimes committed after that date. This applies to every federal offense, not just sex crimes. Federal inmates serve their sentence and are released based on good time credits and any applicable program reductions, not through a parole board. That has nothing to do with the nature of the charge.
State systems vary considerably on both registry classification and parole eligibility for these offenses.