There may be a terms of use clause forbidding splitting like this but it almost certainly isn't a demarcation issue, so there is no real 'bypass' that I am awar

There may be a terms of use clause forbidding splitting like this but it almost certainly isn't a demarcation issue, so there is no real 'bypass' that I am aware of. The demarcation is generally clearly marked (plastic box often called a 'network interface device') and is almost never located at the modem itself. Typically, the legal demarcation is on the exterior of houses. For a multiple dwelling unit (apartment complex) or commercial building, the demarcation is usually inside. ISPs try to choose a demarcation that is accessible from the outside, so as to not require being allowed into the building to test signal leading to the premise. This cuts down on cancelled repairs due to 'no access'. My point in all this is that for most coax and twisted pair services, the indoor wiring is usually the property owners and not the ISP's, especially at the point where it connects to the modem. For someone to add a splitter to wire that is their responsibility and property is almost certainly legal. I worked for one of the bigger ISPs up until last year, so I speak only from my personal experience. For more details: https://en.wikipedia.org/wiki/Demarcation_point https://en.wikipedia.org/wiki/Demarcation_point