I do not think that necessarily follows. Even if you are not the customer, they likely (think they) are designing it for you. Because they need you to keep giving them your data, they need you to find the UI easy to use and easy to hand over data.
The new Google Maps UI is fucking custom tailored to showing you advertisements. It's much less useful now for things like driving directions or finding businesses that don't pay for sponsorship (i.e. if I search for "Rite Aid" from my house, it shows me a Rite Aid 3 miles away instead of the one literally 50 feet from my front door.)
Per Google's 10-K [1], they had revenues of $50 billion on ad sales in 2013, and $5 billion in revenue on everything else. If there is a conflict between advertiser interests and end-user interests, the advertiser will win because Google gets >90% of their revenue from advertising.
If you don't pay for advertising, then you're not Google's real customer. They will do whatever they have to do to keep eyeballs on their products, but only if those products help them sell more ads. See Google Reader - it was neither expensive to maintain nor struggling, but it wasn't as advertiser-friendly as Google's other aggregation products and actively cannibalized users from them, so it went away.
I think we are just interpreting "for" differently, in "the UI design is for the customers". I was assuming a very literal interpration, and saying that UI design is to attract more users, while it seems you meant it in the sense that the end goal of the UI design is for Google to serve more ads. I thnk both are true, and it depends on how "for" is interpreted.