If you go back to the first tall ship race in 1956 you will see the people who organized the race saw it was very popular and formed a new organization. From there the whole tall ship genre has seen quite an evolution but somewhere in this loose classification that now exists was, most likely, profit motive.
I guess that is profit indeed the only justification to call small sailingboats tall ships.
The first race that gave the name to the tall ships was meant to be a revival of the big sailing ships of the past. The original race had two classes, that were maintained for some time: Ships with more than 100T and sailing boats with less. However there were no small sailing boats boats involved on these races and the smaller boats were never called ships or tall ships.
The qualification of Tall Ships appeared in the press to qualify the big ships with hugely tall rigs, the ones of the the bigger class.
Today thew want the biggest possible number of entries in Tall ship races so besides the original Tall ship class they opened the races and participation to practically any boat, even aluminum boats.
For having the boast racing a tall boat ship race the vessels have to be tall ships even if they are small sailing boats. That's pretty ridiculous but what they want is the biggest possible number of entries, ships or small boats alike.
Tall Ship as a qualification for a sailing ship refers to the original meaning, those big sailing ships that raced on the first race, a legacy of the working ships of the past.
Wikipedia explains it well:
A tall ship is a large, traditionally-rigged sailing vessel. Popular modern tall ship rigs include topsail schooners, brigantines, brigs and barques. ...
While Sail Training International (STI) has extended the definition of tall ship for the purpose of its races to embrace any sailing vessel with more than 30 ft (9.14 m) waterline length and on which at least half the people on board are aged 15 to 25, this definition can include many modern sailing yachts, so for the purposes of this article, tall ship will mainly refer to those vessels rated as class "A".
All square – rigged vessels (barque, barquentine, brig, brigantine or ship rigged) and all other vessel more than 40 metres Length Overall (LOA), regardless of rig. STI classifies its A Class as "all square-rigged vessels and all other vessels over 40 metres (131 ft) length overall (LOA)", in this case STI LOA excludes bowsprit and aft spar. STI defines LOA as "Length overall measured from the fore side of stem post to aft side of stern post, counter or transom"
This are what for wikipedia are Ships that can qualify has Tall Ships. This makes sense. They have also a list of Tall Ships.