(In theory all they had to do was assign a CAL to the user's SID, but in workgroups this may not work as expected, so who knows). Well with all the feature updates in 2003 Server, someone forgot to 'finish' the Licensing Code for Per User. Since companies now have to pay for EVERY TS CAL in 2003, this made alot more sense. This would allow 1 Power User to use as many PCs as they wanted while consuming only 1 TS CAL. Microsoft was pressured in Windows 2003 to introduce a Per User licensing mode as it was expected that an XP Client license would no longer include a TS CAL. TS CALs for these devices come from an unlimited pool on the TS Licensing Server automatically. Microsoft made up for this partly in 2000 by letting 2000 Pro or XP Pro machines connect to a 2000 TS Server effectively for free. This means for 1 power user you may have to supply 2 or 3 TS CALs. However in today's world of throw away computers and telecommuting, the reality is most companies will have more than 1 'device' (computer) per employee.
This means if you have 5 people sharing 1 workstation and they all use TS on a server, they only require 1 TS CAL.
In Windows 2000, Terminal Services only included a 'Per Device' licensing mode.