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Try not to wake any sleeping dogs. If I recall correctly, a German man was fined by the tax authority for spending miles earned through company travel. I know most jurisdictions don't have any legislation or case law on this, but on my side of the Atlantic in Canada, FF miles are treated as a fringe benefit and are subject to income taxation.
Frequent Flyer Program
¶ 14. Under this program, which is usually sponsored by an
airline, a frequent air traveller can accumulate credits which
may be exchanged for additional air travel or other benefits.
Where an employee accumulates such credits while
travelling on employer-paid business trips and uses them to
obtain air travel or other benefits for the personal use of the
employee or the employee’s family, the fair market value of
such air travel or other benefits must be included in the
employee’s income. Where an employer does not control the
credits accumulated in a frequent flyer program by an
employee while travelling on employer-paid business trips,
the comments in ¶ 3 above will not apply and it will be the
responsibility of the employee to determine and include in
income the fair market value of any benefits received or
enjoyed.
Source