Within the UK the BBC is not allowed to run advertisements, or promote or endorse products. It's part of their charter. The funding for the BBC comes via the TV License and the revenue generated by BBC Worldwide (the commercial arm).

For viewers outside the UK there is no such restrictions and given that funding for digital ventures by the BBC is now somewhat limited (due to complaints by the commercial sector that they could not compete with the BBC) they try to recoup some money by running advertising.