When one product is sold to a customer only on the condition that something else, usually less desirable, is purchased as well, this is known as a tying arrangement. This does not have to be an illegal act, and there is no one law that specifically outlaws tying arrangements. They were always in the mind of lawmakers who imposed the Sherman Antitrust Act, the Clayton Act, the FTC Act, and others, but trade laws do not by and large prosecute specific defined actions because it is too hard to define the actions. Rather, trade legislation asks whether harm is done.
In the case of a tying arrangement, several issues are considered. Is the party creating the tying arrangement attempting to do something malicious? Does that party have the power (market share or other influence) to enforce a tying arrangement so that the subject of the arrangement either has no choice or faces economic harm? Is the subject of the arrangement in an economically disadvantaged position, or does the subject face economic harm from the arrangement (this is different from the prior question, because the courts can ask separately whether the enforcer or the subject is a basis for prosecuting the enforcer)? Then there are questions such as whether the tying party gains appreciably in the deal, or whether the subject party loses appreciably. Next, can the two tied products be purchased separately, or do they really have to belong together (a left and a right shoe would not be construed as a tying arrangement, nor would the pants and the jacket of a suit, but forcing a store to buy shirts because the store wants to buy jeans might be considered tying)? And there can be many other questions raised.
When a tying arrangement is prosecuted, the court has to examine the specifics of each individual case. Even if the enforcer of the tying arrangement has great power over the subject, if he can't actually cause significant economic or anticompetitive harm, there is no case. If the dollar value or competitive value of the tying arrangement is fundamentally weak, it is hard to prove a case. Any particular case will be tried in part on prior case law that sets expectations for what the courts will tolerate or not, and in part on whether damage is done and to what degree. Sometimes an egregious tying act can prompt a court to find against the enforcer even if it is relatively irrelevant. And cases can be incredibly complex, such as tying arrangements involving purchase of cell phones and provision of service.
Often technical issues dominate a case, as with cell phones tying arrangements (which occur incessantly). The nature of tying arrangements has changed demonstrably over the past thirty years or so; they are less about selling two pieces of merchandise and more about selling service with a product (as in the case of cell phones). This is partly because the classic two-pieces-of-merchandise cases have extensive case law to guide businesses, and partly because technology and services are open to many more ambiguous interpretations of what might be tying. Tying arrangements are not always called such but are often the underlying argument in major international cases involving technology competition.