The rush by cable-television systems to prevent unauthorized viewers from pirating their programs is making it impossible for millions of consumers to take full advantage of the expensive circuitry that was supposed to make their new television sets ''cable-ready.''

Buyers have paid $150 to $200 extra for cable-ready sets, electronically sophisticated televisions that were designed to receive cable programs without the converters that are in standard use. But the technology is only useful when the set is operating off a cable system that does not scramble the video signal.

An increasing number of systems are scrambling their signals to frustrate people who want to plug into the cable but don't want to pay for the privilege of doing so. Thus in Manhattan and many suburban areas where some or all cable signals are scrambled, cable-ready televisions are virtually useless unless they are connected to the same kind of converter that cable subscribers have always had. Cable-ready sets lack the capability to decode scrambled signals.

''The problem with a cable-ready set is that the manufacturers and sellers don't disclose the limitations,'' said Dominick Picardi of the New Jersey Board of Public Utilities Office of Cable Television. ''In some areas, cable-ready sets may work very well. But if you live in a 17- , 23- or 32- channel system area, the chances are its signals are scrambled. So you spent an extra $150 on this set because you thought it would work better, but the extra features are no good to you.''