What are you babbling about? I work in Fed LE. Marijuana cases are rarely prosecuted in federal court unless they are HUGE operations (hundreds, usually thousands, of pounds and hundreds of thousands of dollars worth of product), or unless they involve other violations such as using firearms in furtherance of drug trafficking, tax evasion, money laundering, etc.
+1. Here in Alaska, marijuana possession for personal use in one's residence is decriminalized at the state level, though sales and trafficking are still illegal as is being in possession outside your residence. So, while it bears noting that the new laws in Washington and Colorado are much more sweeping, it also bears noting that the DEA and other federal agencies simply do not have the manpower to try and take up the slack in state level enforcement for simple possession. Actually, like DMF said, they don't get involved much at all in drug cases in AK, the main exception I've seen being interstate trafficking operations (which seems like an appropriate role for federal agencies).
As for the OP's original question about firearms purchase and drug use -- here in Alaska, I'm not familiar with any cases where someone was federally prosecuted for lying about marijuana use when purchasing a firearm (but I work in LE at the municipal level, so don't pretend to have exhaustive knowledge of federal cases, and may just not be aware of a handful of cases where it did occur).
What that probably means, however, is that many Alaskans have simply lied on the forms when purchasing firearms. I'm not excusing criminal behavior, but these are the sorts of secondary criminal conduct you get when a government restriction or prohibition is significantly out of step with the world view of large numbers of citizens -- the same sort of thing happened with alcohol prohibition, with a pen stroke effectively making many (most?) Americans law breakers.
The word I get is that LE is pretty much hog tied on making arrests stick when they suspect someone is under the influence of MM because there is no statutory level.
I'm sure that varies from state to state, but here in AK if the concern is an intoxicant besides alcohol impairing driving, it's not much of a limiter. It does deprive the DA of the very nice piece of evidence that a blood or breath alcohol test provides, but that just makes things like officer observations and audio or video recordings of the defendant from the time of arrest and processing more significant.
Alaska doesn't have a simple public intoxication law, per se, so that isn't an issue here in a way that it might be a limiter on police authority elsewhere.
However, here's the overwhelming misnomer that is being overlooked by the advocates: legalizing marijuana will not eliminate the criminal activities surrounding it, and comparing it to alcohol prohibition is a red-herring argument, as the surrounding scenarios are not even close to being parallels. Tens of thousands of people will continue to be murdered in the processes of getting marijuana traffic around the U.S.. Just like alcohol and tobacco, it is a politically charged topic with literally tons of money attached to it, and it will be bloody.
I don't know that that tracks very logically to me if you mean legalization at the national level. If you mean that some states decriminalizing or legalizing won't decrease drug violence on a national level, then it makes more sense. Here in AK we have occasional murders over user-level quantities of hard drugs, but not so much about user-level amounts of pot (and both are just statistical noise compared to homicides involving alcohol), but up here major grow operations and the trafficking level of stuff is still quite illegal and does result in violent crime (most of our home invasion robberies, for instance, involve people going after dealers for their drugs or their cash).
Nationwide legalization would drain the pond entirely as it results to marijuana -- you might see some initial kickback from the criminal element losing their revenue stream, but the business world would jump on that revenue source like a hobo on a ham sandwich and would not be at all shy about forcing local, state, and federal law enforcement to take action against criminals hampering their legitimate business efforts. Prohibition denies those involved in the marijuana industry recourse to the law for thefts, robberies, etc., and that drives violence. The same was true when alcohol was outlawed, and when the repeal of Prohibition denied that money to organized criminal gangs they branched out into other directions (including the drug trade).
