There is this trick when you give a person two choices: one which you want her to accept; and the other, which is significantly worse than the first one. This way you create the illusion of choice, and also make the first option look better when contrasted with the second.
In some sense this is anti-decoy: a decoy must attract, while this fake option must repel.
Is there a name for this second, fake option?
The fake option might be called a straw man (“An insubstantial concept, idea, endeavor or argument, particularly one deliberately set up to be weakly supported, so that it can be easily knocked down; especially to impugn the strength of any related thing or idea”).
A term that almost works, but probably not quite: stalking horse (“A horse used as cover by a hunter stalking game” or “A candidate put forward instead of any of several potentially successful candidates in order to initiate a leadership debate, gauge feelings, divide opposition etc”).
While plausible and perhaps suitable meanings for “forced choice” and “mental forcing” have been suggested in another answer, note that both terms already have some currency with different meanings; forced choice referring to ipsative questions (“a measure that forces an individual to choose among multiple desirable options”), and mental forcing referring to either psychology-of-magic or to certain beliefs.
This type of situation is often referred to as Hobson's choice (or Hobbs' choice).
It's from Thomas Hobson (1544-1631), a livery stable owner who offered customers the choice of either taking the horse in the stall nearest the door, or taking none at all (to make sure his best horses didn't get overused). Effectively, the "choice" was just take it or leave it.
Thus some people will say the expression implies your only choice is to accept or reject a single option on offer. But in my experience it's also commonly used where superficially there are two or more actual choices available, but all except one are manifestly unacceptable. That's to say, it can be used in situations where there's the illusion of choice, but only one is option is practical.
The best term I can think of for the "fake" alternative is bogus (or sham) choice/option/offer. I don't see that "decoy" fits very well, because usually a decoy is something you're intended to fall for.
In magic, this is called a "forced choice" or "mental forcing"
