Why these URL work in Drupal ?

2 answers.

First, the URLs are served due to Drupals URL path resolution method.
ALL requests are chopped into bits and matched against valid top-level paths. Trimmed back step by step until a match is found.
A request for /node/7/thing/path will fail to match /node/7/thing/path so /node/7/thing will be tried next, then /node/7 . Magic happens here, because /node/7 is not really a path.
In fact just /node gets triggered, and the rest of the path is passed in. The 'node display' function is called, with a request to display number '7' (with extra arguments 'thing' and 'path' which get ignored usually)

Anyway, this means that ANY string you care to tag onto the end of that URL will eventually serve something, even if it's just the normal /node page.
Search Engine fanatics complain about this a bit, but it's not much different from URLs that go /page.php?some=random&string=of#garbage.

Point 2 - why are you getting those requests? It couldbe a random spam-seeding attack, but would in fact be more likely to dilute someone elses rankings by listing themselves in access logs that way.
What probably happened is you just have some malformed links in your own pages looking like
<a href="www.flikr.com">linky</a>
That 'link' is wrong bad html, but happens if the poster is lazy or simply confused. It WILL actually resolve to http://your.site/node/www.flikr.com as a dodgy URL. It will work like a relative link.

Or it's coming out of a really badly coded site spider.

Not worth worrying about, as it's pretty easy to bomb any sites with stupid URLs that may inappropriately resolve to valid pages.
http://www.google.com/search?q=drupal+SEO&spambait=anything/you/like
Drupal ones just hide the ?q= from you.
http://drupal.org/search/node/any/spambait/you/like

S.F.W?

.dan.
How to troubleshoot Drupal | http://www.coders.co.nz/