Earlier quoted context omitted.
"Theft" doesn't just mean "physically taking something away from someone else, who now no longer has it". As much as you might like it to mean that, that's not what the law says.
On top of that, it takes equipment, hardware, personnel, infrastructure, power to deliver cable. The OP literally has no idea what they are talking about, and could use the same logic for electricity theft.
Electricity theft does extract a resource from the grid (electricity provider needs to supply the energy you're using on the other end). Similarly, phreaking uses phone lines which is a finite resource too (and phone company would rather keep those available for paying customers).
Cable on the other hand runs all channels to your household in the first place. By removing the filter, you aren't creating any additional load on the cable network or using any additional resources for free.