> Copyright law only restricts commercial activity
That is just not true. A copyright gives the owner thereof exclusive rights to make a copy of his work. Neither the creator nor the copier have to sell anything.
Noncommercial use is a factor to be considered for fair use if the copier is doing it for a protected purpose. (Creating a t-shirt for your personal use is not a protected purpose and can therefore never be fair use on its own though.)
The reason someone making a personal shirt doesn't get sued is because suing people is expensive, that harms goodwill, and Disney isn't getting any money from such a person anyway.
I should clarify that noncommercial use something different: an exemption for commercial activity. What copyright enforcement can't recover is income from activity that couldn't have generated revenue, in the first place. If I draw a Nintendo character on my own T-shirt, and never sell that T-shirt, that's not a copyright violation, because there's no way I could have made money from it. Whereas, if I draw a character on a T-shirt, then give it away for free, it's noncommercial activity, but it could be subject to copyright, because I could have charged for it.
That is just not true. A copyright gives the owner thereof exclusive rights to make a copy of his work. Neither the creator nor the copier have to sell anything.
https://www.law.cornell.edu/uscode/text/17/106
Noncommercial use is a factor to be considered for fair use if the copier is doing it for a protected purpose. (Creating a t-shirt for your personal use is not a protected purpose and can therefore never be fair use on its own though.)
The reason someone making a personal shirt doesn't get sued is because suing people is expensive, that harms goodwill, and Disney isn't getting any money from such a person anyway.