Nothing will stop them - but, (and I'm just guessing here) - the type of people who host more than one terabyte of photos on flickr probably don't want to screw around with multiple accounts - tracking your detailed stats, alone, gets to be a hassle with multiple accounts. Obviously some people will - but I'm guessing those will be few enough not to matter.

The bulk of the people who want to host 2 Terabytes of Photos will be fine paying $500/year, those who aren't can create multiple accounts. (Those who just want Local+Cloud backups are probably better served by just buying a 2 TB Hard Drive for $90, and backing up with backblaze for $50/year