Not sure why this would have to be the case. Purchasing credit from
Oyster online is a bit problamatic as the site does not take credit
cards issued outside the UK. And if I was able to purchase credit,
would it only show trips from that point forward?

This has been discussed here in the past. I think the consensus was
that it was a security measure to protect the information about where
and when a card was used. (Eg think of partner A suspecting partner B
of cheating.)

Without the need to buy credit online A could register B's card and see
the travel history; and TfL would have no more record of who did so than
an IP address from the computer used.

With the online purchase they also have credit/debit card details and
the fact that the Oyster card was then used to collect the credit. (I
*think* the credit has to be collected before online access is given but
I could be wrong.) Of course the credit card might be stolen but no
system is perfect. And the online credit does give the card user the
opportunity to spot and ask questions about the unexpected balance.
--
Robin
PM may be sent to rbw0{at}hotmail{dot}com