Hmmmm... If it's that easy then it's absolutely silly that these copy-pasta clones start at such a ridiculously low difficulty.
Again, if you have knowledge of how to set the initial difficulty higher please share it. The recent flood of clones is obviously being done by amateurs that probably won't figure this out, and yet will release their coins anyway. So do everyone a favour and educate them. Maybe they will listen.
It is quite easy.. unless a retargeting takes place (which is scheduled in the code every so many blocks) the difficulty for a block is simply the difficulty of the block before..
Hence, the starting difficulty depends on the difficulty of the Genesis block..
All the kids here releasing alt-coins barely know how to generate a Genesis block, let alone change it's difficulty..
The bnProofOfWorkLimit is just that; it's the upper (or lower depending how you look at it) limit of the difficulty.
Edit: To further support the above:
# litecoind getblockhash 0
12a765e31ffd4059bada1e25190f6e98c99d9714d334efa41a195a7e7e04bfe2
# litecoind getblock 12a765e31ffd4059bada1e25190f6e98c99d9714d334efa41a195a7e7e04bfe2
{
...
"bits" : "1e0ffff0",
"difficulty" : 0.00024414,
....
}