if you eat meat, you eat L-carnitine every single day - it is basically like an amino acid supplement. L-carnitine is NOT the problem here, it is the route of administration - infusions have been banned by WADA for sometime unless medically indicated.
Why did they infuse? Easy, the science. It takes 6 months of oral L-carnitine supplementation to get an increase in muscle:
https://www.ncbi.nlm.nih.gov/pubmed/21224234
However, if you infuse it WITH insulin (if they were also playing around with insulin, that is ANOTHER doping issue, as insulin is banned) you can get muscle stores up MUCH more quickly:
https://www.ncbi.nlm.nih.gov/pubmed/16984983
https://www.ncbi.nlm.nih.gov/pubmed/16984983
the rules are clear, you can't use infusions unless you have a TUE, these athletes should be banned...