John Mueller of Google was asked if requiring API users to give the API source a followed link is okay or not. John Mueller said that it is essentially the same thing as a paid link.
He said this on Twitter:
@jsilton That's essentially a paid link. :-/— John Mueller (@JohnMu) August 29, 2016
Here is the specific API terms from Walk Score:
It clearly says that "all use of the Walk Score APIs must follow these branding and linking requirements." One of those top requirements are "logo and score have followed link to How Walk Score Works page." I am not sure why they had to say "followed," they should have said just link and they would be safe.
I do wonder if this will result in a crack down of their links now.
Forum discussion at Twitter.