Technically, I dont think it is illegal to program anything in your radio, but transmitting on any frequency (public safety or commercial) without a FCC license to do so is illegal.
§ 90.427 Precautions against unauthorized
operation.
(a) Each transmitter shall be so installed
and protected that it is not accessible
to or capable of operation by
persons other than those duly authorized
by and under the control of the licensee.
Provisions of this part authorizing
certain unlicensed persons to operate
stations, or authorizing unattended
operation of stations in certain
circumstances, shall not be construed
to change or diminish in any respect
the responsibility of station licensees
to maintain control over the stations
licensed to them (including all transmitter
units thereof), or for the proper
functioning and operation of those stations
and transmitter units in accordance
with the terms of the licenses of
those stations.
(b) Except for frequencies used in accordance
with § 90.417, no person shall
program into a transmitter frequencies
for which the licensee using the transmitter
is not authorized.
Note section b. Yes, it IS a violation of FCC regs to program a radio transmit frequencies in the Part 90 services that the licensee is not licensed to transmit on. (Or allowed by other rules, such as the ones about low-power use on certain frequencies)