I read the following sentence:
あんなおまけイラストまで喜んで貰えて却って申し訳ないです。
As far as I know 申し訳ないです is usually used for formal apologies but translating it like "I'm sorry that you are pleased ..." doesn't really make sense to me. On Jisho it also says it can mean "thank you very much (for help, etc.)", I can't find any examples for this usage however. Could anyone explain me the non-apologetic usage of 申し訳ないです and how that would be translated?
"Sorry", "I'm sorry", "I apologize", and "my apologies" are not always good translations for any Japanese phrases expressing a sense of apology like すまない (すみません), 悪い, 申し訳ない (申し訳ございません), etc. because oftentimes expressions of apology in English (and for that matter a lot of other languages) fail to convey other shades of meaning the Japanese phrases have. They are not equivalents.
Just like すみません can be used to give gratitude to another person or tell them you feel bad in the sense that you don't think you deserve the kind of nice treatment you receive or the other party is doing too much or being too kind, 悪い and 申し訳ない can too.
昨夜も先輩にご馳走になってしまいすみません。
I feel bad that you (senpai) treated me to dinner again last night.
Even the "feel bad" sense has more than one applicable situations. Here's an example:
前回、トルマリン効果がよくよく分からなかったので、自分の分だけ購入してみました。今では一年中離せなくなりました。自分だけ良い思いをしては申し訳ないので、夫と娘のために購入しました。 3年経って私のシートは貫禄が出てしまいましたが、今でも、ちゃんと暖かいですよ。(source)
I felt bad that only I had all the fun using it, so I bought them for my husband and daughter as well.
The "thank you" usage is just an extension, one step further from this. It might help if you translate it as "I'm obliged". So your line roughly goes
あんなおまけイラストまで喜んで貰えて却って申し訳ないです。
I actually feel bad/obliged that you even like the freebie drawing (sketch).
