Why is interest charged from purchase day when I have a promotional balance

I have a Barclaycard and I have taken a deal to transfer some cash to my bank account at 0%. This is all good and fine but when I read the T&Cs I found a clause that said that once I had a balance on a promotional rate any additional purchases that I make will have interest charged from the purchase date even if I paid those additional purchases in full when the statement was due. This is confirmed by this help page:

help.barclaycard.co.uk/brochure/spending_transactions/promotional-standard-rate

I did a test with a small purchase and I was indeed charged a small interest on the purchase even though I paid it in full when the statement was due.

Can someone please explain to me why this stupid rule exists? It clearly dis-encourages me to use the card since once I have a balance with a promotional rate I can't avoid to pay interest in new purchases if I use the card for anything else. Is this a "small print" scam or is it that Barclaycard can't have their systems distinguish from the promotional rate balance and the standard rate balance?

Thanks!