I'm not sure I see how it gives them an excuse: by definition, it would seem that any sideswiping crash during a pass would put them in clear violation of 3 foot laws. I think the problem is just police "interpreting" the law in a way that is favorable to the motorist, not the law itself.

Overall, I don't think these laws are very enforceable, except in case of crashes. The main "good" thing I think could come out of them is some possible publicity/education of motorists about how to properly pass a cyclist. But obviously they aren't even going to be educated if the police won't enforce the law even when it's clear it was violated.

The basic problem seems to be that there isn't a societal consensus that cyclists deserve to get only safe passes, and there is a lot of ignorance on the part of motorists about what constitutes safe passing. Just look at the recent Virginia debate over a failed proposal for a 3 foot passing law: lawmakers there opposed the bill on grounds including that "cyclists are scofflaws, and shouldn't be accomodated", and that the bill would make it impossible to legally pass at all on many roads because of narrow lanes. Apparently cyclists should just be put at risk with close passes rather than motorists be inconvenienced at all (apparently, slowing down and waiting until it's safe to pass isn't an option that entered lawmakers' heads). And motorists should be allowed to take out their aggression on all cyclists because some cyclists break the law. While attitudes like this persist, it's going to be hard to make headway. The only thing that will change this is sufficient numbers of cyclists on the roads, and that will just take time (perhaps with an assist from rising gas prices).