As they say "there is no such thing as free thing".
avast is using the "freemium model" meaning they provide free software (up to a point) but these software will give you ads.
The free software is attracting (potential) new customers and are generating (some) income for avast in the mean while.
And if people are annoyed/irritated buy the ads, they hope those people will buy their software.
In the paid software they say there will be no advertising.
Guess what...
In some of their software there still is if you don't turn it off yourself

About one program that covers all..
Sure it can be programmed that way but that would mean that if someone wants it, it would cost hundreds of $$$ and people are not willing to pay that much.
Next to that people like to have choices on what they want.
e.g.
- If I use webmail only why would I pay for a email scanner when my provider is already scanning it for free ?
- If I already have a good firewall (hardware one), why would I pay for a software one too ?
And as far for the subscriptions, avast is saying they are working on a cross platform licensing subscriptions.
But they are saying that for over 6 months

And the info on the avast account (my.avast.com) is something that avast still haven't got working correctly in over 2 years !
What you see there depends on the avast products and version you are using, the Android versions you are using etc.