Are all "Webster's" dictionaries published by Merriam-Webster?
The short answer is, "No."
The end of the 19th century brought G. & C. Merriam Company copyright and trademark difficulties created by the expiration of early copyrights on Webster's work, the sale of rights to some of his abridged dictionaries, and the expiration in 1889 of the copyright on Merriam-Webster's 1847 edition. The respect that Merriam-Webster had earned for its Webster's dictionaries over the course of fifty years was a desirable asset that unscrupulous companies found they could exploit simply by calling any dictionary they produced or reprinted Webster's.
Merriam-Webster went to court time and again over copyrights and trademarks. One famous suit, lodged against the Saalfield Publishing Company in 1917, resulted in an injunction enjoining that company from using the title Webster's Dictionary without the disclaimer, "This dictionary is not published by the original publishers of Webster's Dictionary, or by their successors." Later suits allowed the use of the name Webster by others, while upholding other marks identifying Merriam-Webster titles.
The net effect of the proliferation of Webster's dictionaries is a reference-book marketplace in which the consumer is either unaware of or confused about what differentiates these books. In an attempt to draw consumers' attention to the issue, the company changed its name in 1982 from G. & C. Merriam Company to Merriam-Webster Inc. and in 1991 reinforced that move by introducing the following position statement to further identify and distinguish its products and to place greater emphasis on a unique tradition of quality dictionary-making:
Not just Webster. Merriam-Webster
Other publishers may use the name Webster, but only Merriam-Webster products are backed by 150 years of accumulated knowledge and experience. The Merriam-Webster name is your assurance that a reference work carries the quality and authority of a company that has been publishing since 1831.