Constitutional Right to Install Bulletproof Glass?
Philadelphia's planned restrictions on bulletproof glass would violate the Pennsylvania Bill of Rights -- "All men ... have certain inherent and indefeasible rights, among which are those of enjoying and defending life and liberty, [and] of ... protecting property ...."
The opening section of the Pennsylvania Constitution (first enacted in 1776, but reenacted since) begins thus:
That the general, great and essential principles of liberty and free government may be recognized and unalterably established, WE DECLARE THAT —
§ 1. Inherent rights of mankind.
All men are born equally free and independent, and have certain inherent and indefeasible rights, among which are those of enjoying and defending life and liberty, of acquiring, possessing and protecting property and reputation, and of pursuing their own happiness.
And Pennsylvania courts have taken this seriously; a long line of cases, going from 1917 to 2000 (see p. 408 of this article), recognizes that this is a serious constraint on government power—for instance, landowners have the right to kill wild animals that are threatening crops, even despite state game laws that bar such killing.
But despite this, Philadelphia seems to be trying to restrict one important form of defending life and liberty (and protecting property): bulletproof barriers that help shield employees from robbers or other attackers.
The just-enacted Bill 170963 would have originally banned them outright in eating and drinking establishments that seat at least 30 customers. (These restaurants are also the ones that, under Pennsylvania law, can get licenses to serve beer, though not all such large restaurants do sell beer; the law seems aimed at "beer & delis," but apparently some local KFCs and Popeyes also have bulletproof barriers.) The bill was ultimately amended to call on the city's Department of Licenses and Inspections to "promulgate regulations to provide for the use or removal" of such barriers—but that Department seems to support broadly restricting them them; it thus seems likely that the regulations will provide for more removal than use.
The original motivation for the ban was apparently that such barriers were seen by some as "an indignity" (perhaps with a racial dimension, since many of the customers of the stores are black and many of the owners and employees are Asian). The Department has also argued that the presence of a barrier keeps the stores from being "places of public assembly" with "a social purpose," and thus should lead them to be zoned differently; it has also argued that customers might be feel less secure in stores where there's a physical divide between them and employees, presumably because the employees would be less likely to help the customers. (The exact theory isn't quite clear to me here, but you can listen for yourself, starting at 5:30 in this video.)
But I don't see how that can be enough to justify preventing store owners from defending themselves and their employees. If you can kill an animal to protect your property, surely you should be able to have a passive, physically harmless barrier to protect your bodily safety.
A reader asked whether the Second Amendment, which D.C. v. Heller suggested applies to "armour of defence" as well as "[w]eapons of offence," might bar any regulations that ban bulletproof glass; I think it might—but the Pennsylvania right to defend life strikes me as an even clearer guarantee.
By the way, these right-to-defend-life/liberty/property provisions are rarely noticed by academics, but have been successfully used in cases throughout the country, in the 21 states that have them in their Bills of Rights. Indeed, even some states that lack such express provisions have relied on precedents from other states that do have the provisions, and thus view the right as constitutionally protected. To be sure, the right isn't unlimited, and likely doesn't invalidate longstanding restrictions on (for instance) the use of deadly force to defend property, or on the use of deadly force to defend life when such use doesn't seem necessary. (I doubt, for instance, that they would do much in debates over the duty to retreat.) But it does have substantial force, and I think they should indeed be used to invalidate bans on defensive barriers.
(For whatever it's worth, OSHA recommends that restaurants "[i]ncrease workplace security," especially for young workers, by "installing … bullet-resistant barriers where appropriate"; but as best I can tell, this wouldn't by itself preempt contrary city ordinances, because it's only a recommendation and not a requirement, and the OSHA "general duty" clause wouldn't preempt the city ordinance, either, see Ramsey Winch, Inc. v. Henry (10th Cir. 2009).)