You (collectively) are missing the points.
IF a boiler owner CHOOSES to send off the warranty card with complete address details, that's up to them. If they choose instead to register 'Mrs Snoggins, <postcode>, Anytown' that's still a perfectly valid warranty registration that they can call on later if necessary. If they choose NOT to register the warranty at all, for whatever reason, that may be unwise but it's their choice. Sales by CORGI of large numbers of site details in bulk to manufacturers is completely different, for the reasons I've already posted.
I also get very ****ed-off with Seagull Service by manufacturers on one- and two-year-old boilers, when a service SHOULD be mostly profit (no parts required). (Seagull: fly in, crap over everything, fly out again with no chance of comebacks.) But so long as it's not made as easy for them as shooting fish in a barrel, I can live with it.