The thing with shirts and sweats at least in the US is that most bands pay a premium to have them made. I’ve seen unit cost as high as 25 for shirts and 40 for sweats. Tack on shipping and theres not a lot of margin. Also bands have to price match with headliners. The venue fees are usually 10% on soft goods. Which is why posters are so lucrative. They kind of skirt by in the corner. Unless its a true indy band, meaning they press the music themselves with no label support, at best an artist can hope to make a dollar of maybe two off an album sale often times way less. Posters on the other hand, unit cost for a screen printed one are around $2. So for twenty bucks 18 is going directly to the artist. Get that thing signed or not and you’ve got an asset that will probably appreciate and not take up too much space in the process.

Of course if one is in a position to and one really wants to support an artist you should license their music or get your company too.