Aliens Steal Cable/Analysis
There are a lot of problems with the idea that aliens could get our Television messages.
First, while an AM or FM audio signal is mathematically easy to decode back into a sound wave, an analog TV signal is very complex; it's essentially a set of instructions to tell a receiver how to shoot electrons at a screen to make David Hasselhoff's rugged suntanned face 25 or 30 times a second. Let's not forget that if you want to see the color of that suntan, there are three ways to go about that: the quadrature amplitude modulation of NTSC (USA, Canada, and Japan), the frequency modulation of SECAM (France, Mongolia, and much of the former USSR), and the quadrature amplitude modulation switching phase every other line of PAL (most of the rest of the world). VCRs, DVD players, and TV sets have to be built either specifically for each of the three television systems or having additional circuitry to be multi-system, or they'll show a black-and-white picture, a rolling picture, or no picture at all. Digital television, which rose to prominence between 2000 and 2010, is even more complicated, involving cosine transforms, motion encoding, entropy codes, and trellis channel coding.
All that complexity and incompatibility is just here on Earth, where all the effort of signal-matching can result in a guy in Omaha, Nebraska trying to figure out what is going on in a Japanese game show that doesn't even make sense to people in Japan.
Unless an alien can puzzle out the nature and workings of an Earth-style CRT, and emulate the behavior of one (as modern sets do to interpret old-style signals) it can't make a visible image out of it.
Secondly, a TV signal is not all that powerful. Add to that the inverse-square law, which says that doubling the distance cuts the signal strength by 75%, and the fact that all stars put out radio waves on every frequency, and it's very unlikely that those I Love Lucy episodes made it to Omicron Persei 8 intact, instead they'd be swallowed up by white noise. Also, since the Earth's atmosphere would make the inverse-square law appear more like the inverse-cube-law or more due to absorption of standard radio frequencies, it is unlikely that any alien receiver, no matter how advanced, could find any information in our TV signals without getting uncomfortably close to orbit...in which case we would detect their presence.
As mentioned in Life After People, television and radio signals which were once thought to be capable of transmitting information over interstellar distances actually decompose into static within one or two light years according to research done by the SETI project. Therefore, if they are correct, aliens cannot steal cable unless they actually come within that distance.
And of course, all of the above considerations aside, this trope still assumes that human languages can even be understood by beings from other planets. This is a major consideration, as even the Pioneer Plaque which was sent into orbit in the 70's to make contact with other races suffers from major anthropological biases such as the use of the arrow to represent direction.