APIs are often not as complete as the web interface, since the customer sees the web interface and normally the customer is what drives the revenue model of the company.
If pages are driven via an API, then the API is preferable, but publicly facing websites are often a mix of server-side HTML generation and API enrichment, for caching if nothing else.
In that case it seems that the webmasters complaining about scraping need to make sure their APIs actually provide access to all the same data, if they want people to use the APIs instead of scraping.
If pages are driven via an API, then the API is preferable, but publicly facing websites are often a mix of server-side HTML generation and API enrichment, for caching if nothing else.