And, going forward, contactless payments introduce another problem in that you can spend more than you have in your account. This is, presumably, because these debits aren't authorised as with a normal payment. If so, not everyone will be given one surely?
The £15 limit protects you and the bank, especially as a stolen card could only (AFAIK) be used three times in any 24 hour period before the PIN is required (a PIN be asked at any time randomly too) - thus the risk of loss from a stolen card is just £45.
I guess that also means merchants don't need to have a constant connection for authorisations when using contactless - which is good for buses. That is until you travel out of London and want to buy a daily/weekly ticket, or tickets for more than one person, that comes to over £15! You'll presumably be told you can swipe your card for up to £15 and have to pay cash for anything else! I can't see most buses taking ordinary debit/credit cards for the forseeable future (coaches excepted).