Expanding on Barry's point above, here is the rule I found pertaining to this situation, excerpted by me for clarity:
§ 74.1203 Interference.
(a) An authorized FM translator or booster station will not be permitted to continue to operate if it causes any actual interference to:
(3) The direct reception by the public of the off-the-air signals of any authorized broadcast station... [snip]. Interference will be considered to occur whenever reception of a regularly used signal is impaired by the signals radiated by the FM translator or booster station, regardless of the quality of such reception, the strength of the signal so used, or the channel on which the protected signal is transmitted.
This seems quite clear in defining the protected signal not by a 60dB contour, but by whether it is "regularly used" regardless of the signal strength. If that's the correct interpretation of this rule it would explain why WKMK is collecting interference reports from its listeners, to demonstate that the interference is occurring in areas where their signal is regularly used.
The rules go on to say:
(b) If interference cannot be properly eliminated by the application of suitable techniques, operation of the offending FM translator or booster station shall be suspended and shall not be resumed until the interference has been eliminated.
Since WKMK has indicated that they will "IN NO WAY will we accept ANY interference to our Thunder 106 signal pattern", it seems to me the only way to eliminate it to their satisfaction would be to change frequency or move the translator to another geographic area.
The protected contour of another station is actually referenced in
§ 74.1204. That section says the FCC will not accept an application for a translator if the proposed operation would involve overlap of predicted field contours with any other station. But since the application process is already finished that rule should no longer apply. Instead, I think you would need to refer to
§ 74.1203 above for the rule concerning actual interference once the translator is on the air.
At least that's how I read it, but then I'm not a lawyer.
If you would like to read the whole thing for yourself it is
here.