In foreign languages with a polite form of "You" (such as French or German), there is an additional question: should a website use the polite of familiar form.
And it's widely accepted that a website should use the polite form.
And it's widely accepted that a website should use the polite form.
There are really interesting problems that you come across when you build multi-lingual interfaces.
Why would a computer/website be polite to me? They have no concept of these things. Just using polite forms doesn't mean you're being polite, you have to mean it. A computer is as of yet incapable of having intentions.
The computer isn't being polite to you, the company whose website you are on, is. In the same way your computer isn't being casual and friendly with you - the person who wrote you the email is.
Whether it's an app or a website, it's all just a conduit - and I don't think it's farfetched to say that by and large companies use polite forms when referring to their (potential) customers.
More business-y sites tend to use the formal "Sie" though, including most ISPs and telecoms.