I think there's a different force at play here:

When the software comes with the computer, you can turn around and complain to Dell about the software. There is someone you can call, and generally it's rolled into the phone support contract.

When the software is installed afterwards, you don't have the same ease of complaining. Dell can argue that Firefox somehow messed up your computer.

That is a convenience many people are willing to pay for. Whether they would pay the 16£ is a different question