As far as I can tell, this is in fact a draft of a bill which would decriminalise marijuana.
It has not been passed, and even if it eventually is passed (likely in a changed form), it will not make possession and sale legal.
If you legalise and apply tax the government would see additional sales tax revenue but also a reduced corporation tax revenue.
I imagine they'd still be better off by a long shot because they'd likely spend less money fighting drugs related crime, and also because legalisation seems likely to increase consumption.
Since when do criminal gangs pay corporation tax?
edit: I'm not sure I'm explaining myself very well so let me just spell it out:
If it was legalised and taxed they would receive sales tax income (VAT or whatever they call it in greece), and probably additional tax income from likely "harmful substance" taxes (like those applied to alchohol, cigarettes, petrol etc). At the same time the folks who had been selling drugs illegally would lose income and thus reduce their own spending in the economy.
The result would be increased revenue for some businesses (who started legally selling the dope) and reduced revenue (& thus corporation tax) for all of the other businesses who didn't.
http://greece.greekreporter.com/2011/08/02/decriminalization...
It's still a good step, since it doesn't get you a criminal record nor it clogs the courts, but it's not anything near to actually legalizing and regulating the sale and consumption.