Because DeviantArt is a bad website.
More specifically in this case, DA requires a user to be logged in to be able to access any image flagged as "mature". As this site isn't a "user", DA tells the image grabber to pound sand despite its purported oembed support.
If you do not specify a field to search over, the search engine will search for posts with a body that is similar to the query's word stems. For example, posts containing the words
spread wings would all be found by a search for
sewing would not be.
|Literal||Matches the author of this post. Anonymous authors will never match this term.|
|Full Text||Matches the body of this post. This is the default field.|
|Date/Time Range||Matches the creation time of this post.|
|Numeric Range||Matches the numeric surrogate key for this post.|
|Full Text||Matches the title of the topic.|
|Literal||Matches the numeric surrogate key for the topic this post belongs to.|
|Numeric Range||Matches the offset from the beginning of the topic of this post. Positions begin at 0.|
|Date/Time Range||Matches the creation or last edit time of this post.|
|Literal||Matches posts with the specified user_id. Anonymous users will never match this term.|
|Literal||Matches the short name for the forum this post belongs to.|