If I find a public web API (by public I mean that isn’t protected by any type of authentication) what do I need to do so that I am allowed to use it in my own application?
Is it enough to read and follow the terms of service, or do I need to do something over and above that?
Is it true that this is as legal as using a web scraper and that all search engines get away with that?
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I am not a lawyer and you aren’t even saying which jurisdiction applies to you, so no legal advise from me.
But from a purely practical standpoint, building a product based on an undocumented API is a very bad idea. You have no way to tell if the API will still be there tomorrow. The one who made it can destroy your whole work with a single change to their API.
They might do this intentionally because they want to keep you out. After all, you are only consuming their resources without giving them anything in return. Or they might do it just because their own requirements changed and they don’t even know that others also use their API. The only way to have any chance to get a warning in advance is to ask them if you may use it. When you are serious about your project and their API is essential for it, you could ask them to make a deal that they guarantee availability and stability of their API in exchange for money.