Continental is based out of the UK so it'll be mad expensive to get things shipped over here and print on those. Most people just buy bulk shirts and get them printed on and cut out the tag and replace with their own personal tags. For high end tees people usually buy American Apparel tees and then cut out the tag, and print on them and bam done. Legally I think you can print on anything you buy and then resell it. It's not like you'd have to pay American Apparel for the right to print on their tees. Alot of bands and musicians print on American Apparel tees and nothing happens to them.
Most of the tall tees that are put out are either made by Fruit Of The Loom or Hanes. Even the ones at Foot Locker or Finish Line or whatever, usually those are FOTL or Hanes, and all they do it cut out the tag and put their little stamp in for the sizing. FOTL and Hanes dont care because they are in the tshirt making business not the printing business. Hope this helps.