Earlier quoted context omitted.
Why would you feel bad? The whole point of open source is they could have done it themselves, if they wanted some integration they needed and you didn't. And then they could either contribute it back, fork your project or just keep it to themselves, if the license allows it. At any rate, you already helped them...
People often act very entitled about open source I've found. You'll often get angry irate emails/issues raised demanding you help them or add a new feature or whatever. Some people are just clueless and need help, others are just dicks. It is often not worth the hassle in my opinion, but then everyone's circumstances and motivations are different and I am glad that a lot of people do think it is worth it.
It's interesting. For me there are at least two classes of open source.
1) Project with a single maintainer, or perhaps a small number of key people, nothing in the way of corporate backing, often worked on in spare time, but which can still be very popular and widely used. I always think back to the example of NDoc, which was a great tool for taking raw XML doc comments from .NET code and turning them into beautiful Javadoc-style HTML documentation. It worked incredibly well for .NET 1.x but the .NET 2 support was never finished because the author, Kevin Downs, received abuse and threats via mail-bomb due to it not being ready "fast enough" for some people. Understandable he decided to quit and, as far as I'm aware, not a peep has been heard from him since in terms of OSS contributions. The way Kevin was treated, and the way other OSS maintainers have been treated by both individuals and corporations, is absolutely despicable and completely unacceptable.
2) Corporate backed OSS projects that are actively evangelised: projects like .NET Core, Node, npm, TypeScript, React, Angular. You might even think of something like Firefox in this category since, although Mozilla is a foundation, it's substantially funded by corporate sponsors (mainly Google?), has full time employees paid to work on the products, etc. Some of these might at times have been considered open source in name only: i.e., source code is absolutely available, but there is little or no way for most people outside of the sponsoring organisation to meaningfully contribute. With these I take a slightly different attitude, and I certainly expect sponsoring organisations to take more responsibility for the projects. If you're actively evangelising people to use a project (and especially if you've succeeded in recruiting large numbers of people to use your project), and you're not giving them much opportunity to contribute, then absolutely you need to take responsibility for making sure the project is supported appropriately. That might even include paid support options.
And I suppose perhaps these represent the extremes of a spectrum (perhaps - I don't claim to have this absolutely right, by any means).
Clearly there are a lot of people who would disagree with me on both sides. E.g., people who think every open source project should be like (2) and that all authors bear an equal responsibility for supporting their code. On the flipside, since I've been flamed on this before, I know there are people who think I'm a walking manifestation of all human evil because I've suggested that any open source project might perhaps fall into category (2).
Fundamentally though I think your point stands: a lot of people behave in a way that's very entitled towards individual maintainers of OSS projects. As you say, depending on your circumstances, often not worth the hassle.