Non-compliant redistribution defeats the point of open content. Other people who see the unlicensed content will not know who wrote it, will not know that they have the right to obtain the source, will not know that the content comes from Wikitravel and will not be able to contribute their knowledge.
Non-compliant redistribution violates the copyrights of the original authors. They have agreed to license their work under the specific conditions of the CC by-sa 3.0 license only, and the copyright holder can, legally speaking, sue offenders for copyright violation.
Non-compliant redistribution should be addressed by contacting the distributor in question and gently and politely noting their obligations under the license. Pointing to the Wikitravel:How to re-use Wikitravel guides page is a good way of showing them what to do and how to do it, and most people are happy to comply if asked nicely.
Borderline or unclear cases, cases that have not replied to contact attempts or cases that have rejected attempts to comply should be posted on the talk page.