There's nothing at all wrong with linking to sites that require a subscription if you're recommending the content. It's not "soliciting business," it's the way the internet works. Some sites have ads, some have subscriptions. *shrug*
The Heuschmann video about the biomechanics of the walk "Stay in Step-The Rhythm of the Walk" is also available in the subscription area. I personally thought the information was well worth the subscription. It's the clearest explanation of walk mechanics I've seen or read. I haven't seen this particular video available elsewhere, and it's not the same as his DVD.