The States could have assigned the task of ex-felon data collection to a law enforcement agency or allocated funding for a private business to perform that function. They did not do so, the cost in tax dollars would have been enormous. Instead, the State imposed a duty of self-reporting on the individual offender to alleviate the fiscal burden. The State remains obliged, however, to provide compensation for the services they have accepted. See, e.g., United States v. Russell, 80 US 623, 630 (1871).
Sex offender registration laws compel a registrant to maintain a residence, take time off work, travel to a registration facility, purchase photos and fingerprint cards, fill out and verify a registration form and pay postage for mailing the from to the Registry. Although the frequency of these acts can be reduced if the registrant rents a home and does not move, change jobs or alter his appearance, the burdens of registration remain: assist the State in keeping its web site current within a  hour margin of error, for a term of years or for life, or go to prison.
A similar duty of self reporting is imposed on all ex-felons. Although less rigorous than that imposed on sex offenders, ex-felon registration laws are nonetheless a burden and of considerable benefit to law enforcement and the general public.
Labor is defined as work of any type, including mental exertion. Black's Law, Seventh Edition. Compensation for forced labor is fairly assessed by considering its value to the [State]. See, 18 USC 1593(b)(3). A private detective, for example, would have to locate a Registrant, collect and verify the Registrant's personal information, and transmit any changes in that data to the Registry every  hours in perpetuity. This would no doubt run into thousands of tax dollars per week, per registrant.
Because the Registry is of such benefit to public safety, tax payers and law makers appear estopped to refuse reasonable compensation for the services they have demanded. Moreover, an award of compensation to an otherwise exploited subclass would encourage compliance while a lack of compensation would encourage non-compliance.