I disagree we are being ripped off when you consider raw price on like for like devices and remember, you can always use a TV as a computer monitor (With the exception of i resolutions (e.g. 1080i)... these come out terrible).
However, as others have pointed out here, it is rarely like for like - the majority of the time, larger computer monitors have higher resolutions.
After this, if you really are comparing a similar specification monitor to TV, you should also take in to consideration a few additional facts such as warranty.
A TV usually comes with one year and is "designed" to be used for a few hours a day where as computer monitors usually come with 2-3 year warranties and the high end models (such as 25"+) are expected to be used for the majority of the day.
A further example to understand my point is to look at public display televisions - again, spec for spec, these are much more expensive than regular TVs but use higher quality components* and are rated for 24x7 usage.
(* component quality is subjective... I personally think they are very similar, but have a warranty to back it up - unless someone does a tear down, I doubt many people will know).