Totem pays for the Bandwidth and Servers, so it's not Free..
I thought the demos were P2P and, therefore, free. If the demos cost Totem bandwidth, then agree there is a cost issue and that may require some limits.
If Totem does decide to limit demos, I can see two basic alternatives:
1. Limit the total number of demos that can be downloaded based on the total number of cards purchased. example: 40 demos plus twice the number of shows you own.
2. Limit the number of demos that can be downloaded in some period of time like a month based on the number of cards purchased in some period of time like the current plus prior month. example: 10 demos per month plus three times the number of shows you bought since the start of the prior month.
They could embellish this by charging, say 2 credits (new price scale = $.20) for each demo above the set limit. I'm assuming that a demo is 5-10% the size of a full show and the download cost to Totem is proportional.