Actually there wasa law that affiliates can preempt any program they wanted to and the networks had no recourse except to put preempted shows on another station in the market or pull affiliation and go to another station altogether. Though Networks could atempt a contract not allowing preemptions this was illegal. The reason was that the local owner or company that owned the affiliate was to control the station and not the network. Until 1984 companies including networks could only own or control 7 TV stations. So a no preemption agreement would constitute control of many additional stations. After 1984 the amount of stations were upped to 12 with partial control allowed for 14 stations - still a one to a market rule. Then in 2000 companies could own 2 stations per market.
So yes an affiliate was allowed to preempt any show they wanted to. Networks cared more in some places than others. In Cincinnati for example if 12 WKRC TV preempted a show it was likely 7 WHIO Dayton which still had a grade A signal in Cincinnati would likely run it. But sometimes a show was preempted in 2 neighboring markets but that was likely a low rated offering. Most shows with even decent ratings would run in a neighboring market. Cable systems all carried duplicate network stations prior to the mid 80's and in some cases into the 90's and in some cases even today. So cable often corrected any preemption issues that mattered.
So no preeemption agreements were not considered legal because at issue would have been Who controled the station. If ABC for example had agreements with affiliations to never preempt programming that would have given ABC control over at least 120 stations which was not allowed.
So as a result if a show got high ratings, IT RAN, medium ratings it USUALLY ran, low ratings - MAYBE or MAYBE NOT. But affiliates needed networks as much as networks needed affiliates. Affiliates could only produce so much viable local programming and buy only so many shows in syndication to occupy the day. They needed fresh programming so the networks served them well in that area. The independents relied haevily on older shows and old movies while affiliates really wanted to stay away from that stuff except in small markets maybe.
Since you mentioned Cincinnati NBC doesn't seem to have been too intolerant of WLWT's pre-empting network shows in favor of Paul Dixon and Bob Braun. Quite a few NBC daytime shows seem to have ended up on either WKRC or WXIX, and at one point WKRC was carrying NBC's Wednesday Mystery Movie late Saturday nights because WLWT carried its own block of movies on Wednesday.
And if you're number one in your market, you can get away with it; the three most pre-emption happy stations I ever saw were WSB Atlanta, WBRC Birmingham, and WTVT Tampa-St. Petersburg, and all three were number one when I lived in those markets (although I think NBC had tired of WSB's pre-emptions, 450 hours' worth in the four years prior to the switch to ABC).