I thought about it all night long, I couldn't sleep at all, but I think i finally got the joke. Hang on.
Why the hell do the crab order 2 drinks ?
Let's think about it for a sec...he orders a scotch and a pint of lager, right ? That means he will have one in each hand. (1)
If his 2 hands are busy, there is no way in hell he could climb up the ladder, especially if another crab is there trying too. (2)
And as you can all notice the bartender seems pretty annoyed by the crab. He clearly tell the crab that he won't serve him. (3)
(1) = I am french and I don't know the english word for the crab's hands.
(2) = I'm trying to visualize this, and I find it rather pathetic.
(3) = The bartender is crab-racist.
(1) + (2) + (3) = French + Pathetic + Racist.
I think I got it now.
This joke is then a uk-private joke about french people being pathetic and racist.
Somehow I don't like it at all
-- Masklinn --
PS : Thaz the better explanation i could get