It might be because 可愛い looks like it should be pronounced かああい, so the furigana serves as a reminder.
After some quick research into what determines whether a word has furigana, it seems to be dependent on the publisher or author. Furigana is always used (at least for the first appearance) for words with irregular or unclear pronunciation, such as gikun or names.
For similar reasons, 停める regularly includes furigana. According to jpdb, this kanji is used roughly 2% of the time for とめる, making it a rarer usage. This can happen with other characters too. I’ve seen to drink, usually written as 飲む, written as 呑む with furigana indicating the pronunciation as のむ. It can also be seen in old character forms, such as 嚙む having furigana to indicate its pronunciation is the same as 噛む.
It seems books may choose to include or omit furigana based on the target demographic. A book intended for adults probably would only include furigana for rare non-jouyou kanji. For example, 漕ぐ (row, peddle) is not a jouyou kanji, and regularly has furigana. It may also be decided by kanken levels, a kanji proficiency exam taken by Japanese students.