There are a half dozen other mass ad crawling services selling the same service (being able to search and analyze your competitors' ads), dating back at least several years.
On one hand you'd think if this wasn't permissible, they'd be big targets and Google would go after them legally. On the other hand, what would the legal basis for blocking this be?
Someone browsing my website and seeing a Google ad has not agreed to any Google terms of service. It's a tough argument that a browser is allowed to do this but a crawler isn't, especially when Google's own crawlers now include javascript-executing webkit browsers.