I know, for a fact, there are things imposed on Lulu by Apple, Amazon, B&N, Kobo etc. via the Marketing Agreement, simply because I can set any price I want for an e-pub if I load it direct to the Amazon site, and can do the same with just Lulu, but go through the Marketing Agreement and the price must end in .99 cents, and the MA system will adjust it for the other sites while leaving the price I set at Lulu alone. Thus it's an external imposition by the software you have to go through with the MA to use the other sites.
I now Lulu will allow me to ignore the capitalisation rules as long as i list for Lulu only, that's with both e-pub and print book. The moment I list for the others the software kicks up a stink about the capitalisation. I tried it once to see if I could sneak it in, but couldn't.
I don't know which mob insists on the rule, but it's part of the MA rules imposed on Lulu.
Now, as to the rule itself, it's a very old style manual rule that you do not capitalise conjunction in a title unless it's the first word in the title. Where it comes from is beyond my knowledge, I just know that's how it was taught back in the 1960s and was in style manuals I used for work in the 1970s.
edit to add:
Rule 16a. Composition titles: which words should be capitalized in titles of books, plays, films, songs, poems, essays, chapters, etc.? This is a vexing matter, and policies vary. The usual advice is to capitalize only the "important" words. But this isn't really very helpful. Aren't all words in a title important?
The following rules for capitalizing composition titles are virtually universal.
Capitalize the title's first and last word.
Capitalize all adjectives, adverbs, and nouns.
Capitalize all pronouns (including it).
Capitalize all verbs, including the verb to be in all forms (is, are, was, has been, etc.).
Capitalize no, not, and the interjection O (e.g., How Long Must I Wait, O Lord?).
Do not capitalize an article (a, an, the) unless it is first or last in the title.
Do not capitalize a coordinating conjunction (and, or, nor, but, for, yet, so) unless it is first or last in the title.
Do not capitalize the word to, with or without an infinitive, unless it is first or last in the title.
Otherwise, styles, methods, and opinions vary; for instance, certain short conjunctions (e.g., as, if, how, that) are capped by some, lowercased by others.
A major bone of contention is prepositions. The Associated Press Stylebook recommends capitalizing all prepositions of more than three letters (e.g., with, about, across). Other authorities advise lowercase until a preposition reaches five or more letters. Still others say not to capitalize any preposition, even big words like regarding or underneath.
Hyphenated words in a title also present problems. There are no set rules, except to always capitalize the first element, even if it would not otherwise be capitalized, such as to in My To-go Order (some would write My To-Go Order). Some writers, editors, and publishers choose not to capitalize words following hyphens unless they are proper nouns or proper adjectives (Ex-Marine but Ex-husband). Others capitalize any word that would otherwise be capped in titles (Prize-Winning, Up-to-Date).
and another link