This has been coming a long time. The updates were already very cheap (20something Euros) compared to the old days (> 100 Euro if I remember correctly) and it fits in with the iOS strategy of having the latest version on every device.
OS X upgrade revenue is now such a minuscule piece of the pie that it probably doesn't matter anymore.
> the old days (> 100 Euro if I remember correctly)
Yes, 129 for single-user and 169 for family (5-user).
And these are not so old days, Leopard was 129 and that was in 2007. Snow Leopard and Lion were 30, and ML was 20 (in USD, IIRC $129 translated to 129€ but ML is "only" 18€)
Apple's policy until System 7 was that you paid for the hardware and you should be able to expect software updates free for the life of the product. Nice to see them coming home, on this point anyway.
OS X upgrade revenue is now such a minuscule piece of the pie that it probably doesn't matter anymore.