i think both are names, nothing more. a monitor could be called a bookshelf, & wice-wersa. historically, the term "monitor" meant for use in a recording studio, but now, it's also referred to for stand-mount speakers used in a home enwironment. "bookshelf", to me, is an outdated term, from the 60's, or earlier, which referred to a smaller more "home-friendly" speaker that could be placed on a bookshelf. any "bookshelf" speaker will still most likely sound a lot better if set on a stand, out into the room, away from walls.
i think it is inaccurate to say monitors are cheaper and have more toys, etc. actually, cuz the term "bookshelf" is so outdated, you will find the most expensive smaller speakers are likely called monitors, whether they are for use in the home or in the studio. and, the same goes for cheap speakers - they will likely be called monitors, regardless of cost. but, if anyone is adwertizing new speakers as "bookshelf speakers", then i would hazard a guess they are looking to sell to non-audiophiles, who are more interested in squeezing a speaker into whatever space they can fit it - sound would likely be a secondary consideration.
doug s.