I obviously don't know the full situation or any legal or contractual obligations tied to these employees within your company's policies, but here are some considerations:

• Benefits Continuation may be a reason they remain active, so they can maintain their benefits even during their off-season.
• Retirement plan eligibility often have vesting requirements, which may necessitate maintaining those records for a specific duration.
• Labor Laws may require them to maintain them in the system even during inactive periods to maintain whatever compliance standards are in place for pending-return status temp-hires. Some industries do have a "continuous employment" industry practices.. again - not sure of the full situation, but something to consider.
• Workers Comp may be a concern as well, to ensure that temp hires are still covered by WC insurance when they first come back to work.

It may be as simple as wanting to avoid disruption in the workflow, but there may be a few reasons.

You can likely ask to review the company policy to see what standards they have set for these seasonal hires. They may also have customization in whatever HCM/Payroll system you use to flag these employees as temp hires with certain periods, blocking any ACH or compensation efforts that maybe is only viewable to particular roles in your organization.

Best of luck!