Are they restricted by law or is it by their own choice?
By law? Maybe, maybe not. But liquor licenses are insanely expensive- truly a barrier of entry into the liquor business. In particular, liquor licenses that allow consumption (bars, restaurants, any place that serves). The licenses come with a long list of restrictions, usually tailored to the specific establishment- what can be served, where it can be served, and precisely where a drink can be taken.
So, for example, if you try to walk out of a bar with a beer in your hand, that may well be a violation of the terms of the liquor license, and that is why the bartender will likely come screaming after you to ditch the bottle. His license is now in jeopardy. Liquor licenses (at least here where I am) are by requirement owned by individuals, who are individually accountable for infractions (penalties, etc). No corporate veil. For example, my condo has a liquor license, but the license is actually owned by a couple of old time owners, not the condo association that runs the liquor concessions, because that is the requirement of the liquor board. And, of course, in addition to penalties is the threat of the loss of that very valuable and expensive license. If some renter (this is a summer vacation condo operation) walks off the recreation deck with a drink or a bottle in their hand, those owners, who may not even spend any time there, are personally responsible. And the licence in jeopardy.
I went into this for two reasons. First, to suggest how onerous regulations can be, in this case a liquor license. And second, to suggest that this is one possible direction vaping enforcement could head, at some level. You might argue not likely, but it *could* be done. At some level, maybe not to the extent of the liquor biz.