I completely fail to see the logic where the input of a serial number is any different, conceptually speaking, from an online activation. Could you please enlighten me?

If your concern is related with the security of the data that you have to provide when you activate it, this is normally minimal (name and email address). Where's the problem in letting know the manufacturers the name of whoever is using their software? Is it a matter of principle?

Furthermore, if you used a credit card to buy a software (or whatever) than it's already recorded somewhere (and therefore available to someone) that you purchased that product. What's the difference between that database record (i.e, person X bought product Y) from the activation record database (i.e, person X activated product Y)? Unless you don't use credit cards also and buy everything in cash! LOL :-)

The issue raised by LCBoliou is a completely different matter: subscriptions instead of (or in addition to) an initial payment. That was not the original topic. With this, I also don't agree because I may want their product only and not their service. As a consumer, I should not be forced to buy a service I don't want. But, again, this is a completely different matter from online activation.