Lets start with something simple here: I'm not contending in any way that anyone owes me anything. I'm asking whether it wouldn't be better to spend the money on building a simple web application instead of focusing on these two platforms.
Since you're here and you've taken the time to write such a thoughtful reply I'll address it directly. If I had donated money to the kickstarter campaign, they would indeed "owe me" an app for all senses of "owe" that matter for conversation about the apps in question. Then, if I or someone I cared about wasn't an owner of an Android or iOS device, it's reasonable to wonder why the campaigners can't achieve the same result using a technology that works for a significantly larger market of devices (including desktops).
Let's continue with simple thoughts, shall we. Presumably before donating to the campaign you would have determined that it met your requirements. If your requirements included a minimum level of device coverage then you would have withheld your support.
If you did not donate to the campaign then why should the developer be criticized for not meeting your requirements? They don't owe you anything. Including meeting your expectations.
Added: If they left a need on the table then take it and run with it.
Usability is also an accessibility concern. If you make something that is technically accessible to anyone, but difficult to use (because mobile web is harder for people), I'd argue you are cutting out more people than you are by limiting to Android and iOS. And this application is explicitly mobile anyway, because you can drive along and have it show you what's what, rather than memorizing the raw info ahead of time. That is, you can at least make the case that creating a great experience for a large majority of people will reach more net users than a poor to mediocre experience in a platform-agnostic way.
