The problem with web apps though is that they aren't really local to your computer. We have all kinds of sandboxes to prevent, say, a web page from opening up

The problem with web apps though is that they aren't really local to your computer. We have all kinds of sandboxes to prevent, say, a web page from opening up a connection to talk to your PCI cards. I wouldn't say that this is the same as giving battery status to every app on your laptop. A closer example might be smart phones. But there's a presumption that if software is running on your laptop or smartphone you have at least some idea of what it is and what it can do. In practice not so much and such apps could misuse this data along with others.