to be fair, if I was baited as a reader for a ship I love to turn out to be another ship-- I'd probably be upset
not to mention how jarring it must be to have the pairing in the book not match the cover. like I'm not particular about shipping, but if a book has Kasumin and Yuu on the cover, and it's a Kasumin x Shizuka book, I'd be thrown for a loop (also makes things harder for those who do want the contents but then passed it by because of the cover)
but I am familiar with the rage and chagrin that can result. while I may not be particular about shipping, I am about genre. and too often I've found books with two cute girls on the cover, just to open it and find a undisclosed guy having a harem or cheating with the two. like ugh
worst one was a standard isekai harem using the yuri yin yang on one of the later volumes