S.D. Codified Laws § 49-31-4.3
Each telecommunications company engaged directly or indirectly in any business other than that of providing telecommunications service shall keep and, if requested by the commission, render separately to the commission, in like manner and form the relevant accounts of all such other businesses. The provisions of this chapter apply to the books, accounts, papers and records of relevant transactions with such other businesses. All profits and losses of such other business may be considered by the commission as are relevant to the general fiscal condition of the telecommunications company. The commission, in determining the allowance for materials or services to be included in costs of operations for rate of return or price regulation for noncompetitive services, may disallow any unreasonable profit made in the sale of materials to or service supplied for any telecommunications company by any firm or corporation owned or controlled directly or indirectly by such company or any affiliate, subsidiary, parent company, associate or any corporation whose controlling stockholders are also controlling stockholders of such telecommunications company. The burden of proof shall be on the telecommunications company to prove that no unreasonable profit is involved.
SDCL 49-31-4.3