As with anything with domains, usage plays an important part in regards to infringement. It does not matter what other sites do, all that matters is how you proceed with the domain you have. Do you know if these other sites got permission from Google or not? Do you know if Google evens knows about them? Do you know if Google knows aobut them but sees it will not hurt them, so they decided to not pursue the matter?
Disclaimers are only good when used in conjuction with other factors (mainly usage). It is also up to Google to decide if it is worht theri trouble or not if they do find out about it.
Using my previous analogy, if you do 56 in a posted 55 mph, are you breaking the law? The answer is yes. Is a cop going to stop you and enforce that law? no. I know where I live, a cop will not pull anyone over unless they do in excess of 10mph over the posted limits (except for school zones, they are very strict with that). So if you do 9 mph over the limit and you pass a cop, he won't stop you. True story here, I saw a vehicle pass a cop car who was doing radar, the vehicle was doing at least 80mph in a 55. The cop did nothing. Why would this be? It was an ambulance... So there could always be many other factors that decideds enforcement or not. But using the "they do it" defense in never a good one.
Anyone who has been here a while knows my point of view on fair usage and how strongly I believe in it, the main thing is, use the domain in good faith and I will back you 100%, if you do not use the domain in 100% good faith, except me to ride you.