So (@dangrossman) the legal issue is averted: both costumers are in the same isp which, to them, works like a black box. So the article's use of the word "steal" is quite nonsensical.
Also remarkable is the advanced anticipation of demand. For example, Netflix is not even available in Spain and bandwidth demand is currently low compared to say the US. My guess is that it is not to facilitate video over the internet etc. but that it is a cheap way to undercut competition from Vodfone and Orange (other Spanish providers). Bandwidth sharing might mean more consistent high bandwidth for those who need it.
Actually the potential issue is inverted: you may not be able not opt out of the system.
The other potential issue is Telefonica itself. Historically it has been as appalling company to customers. It is possible this is changing but there certainly used to be a long track record of awfulness. Indeed it is my utter shock that Telefonica of all companies on earth would manage to do something of this sort that is compelling me to ramble on like this.
If Telefonica is changing, kudos!