What CBS fears most.

WHEN CHEVRON bought Gulf and Texaco bought Getty, it was often asked: Why don't these companies drill for oil, rather than buy it? (Although, of course, neither party's exploration budget necessarily declines in an oil-company merger.) But no one asks Ted Turner why he wants to buy CBS instead of starting a fourth broadcast network. Anyone can get into the oil business, but it's practically illegal to get into the network-television business. That's why CBS is even more vulnerable to deregulation than it is to a takeover.

For years the Federal Communications Commission proected the networks from potential competition. If limited the number of VHF stations in each market, with the usual ceiling set at three, even though more are often technically feasible. It retarded the growth of cable, subscription, and pay-per-view formats, and required cable systems to carry the signals of their local over-the-air competitors (the "must carry" rule). It passed out low-power TV licenses in such a way as to make it hard to string together a network from them.

Starting under President Carter and more rapidly under President Reagan, the FCC has begun to reverse this process and open up the industry to more competition. Already Turnerhs Cable News Network is a serious rival to the broadcast networks in some respects. But since many cities are still not wired, cable networks cannot offer advertisers truly national audiences.

Which helps explain both the motive for Turner's bid for CBS, and the difficulty of achieving it. So long as the networks enjoy their cozy status it will be hard to rally investor discontent against them, although the stockholders-be-damned attitude of CBS management may yet succeed in prompting a revolt. But full deregulation would make it easier for Ted Turner to get into your living room whether or not his bid succeeds--and, incidentally, would make it less important who owns CBS. The prospect network executives fear most is not that of having to put on George Will for balance, but that of losing viewers by a turn of the dial.

The deregulatory agenda might start with urging the FCC to "drop in" the maximum number of new stations consistent with avoiding technical interference; getting rid of "must carry" and other federal restraints on cable; and, perhaps most important, loosening the political strings local governments have been wrapping around cable operators. The idea should be to secure as many different cable channels free of local-content requirements or other controls as customers are willing to pay for.

This alternative also has the advantage of being consistent with conservative principle, unlike resort to the Fairness Doctrine and other ventures in government content control. Combatting the consequences of network monopoly is a hopeless task. It's time to combat the source.