In message , asdf
writes

What about all the tourists? It's hard for many of them to understand
Oyster. Plus it would waste their time, and increase TfL's costs, if
everyone visiting London (even for a day) had to buy an Oyster card,
use it, then get their deposit refunded at a staffed ticket window.

That's why the Oyster Visitor card exists (no deposit, although there is
a small fee to cover the cost of posting it out in advance).
--
Paul Terry