...so in short
you were looking for a lynch mob to join you in lynching every shop employee who stops people from taking photography.
yes, i know there are dumbos who just do it for the sake of making their day. the point is, why be so uptight about it? life is better when you live it with an open mind. you are not wrong to feel irritated. you are not wrong to feel that you should have the right to take photos nonetheless. i agree with that. what i disagree with is your reaction and methodology of handling the matter, and the fact that you are giving all sorts of justification for that over-reaction which you displayed.
we apologise for not being sheep who are orchestrated to your cause and banner, oh emperor sith. i apologise especially for the fact that i am pointing this out.
why can't everyone just be nice? if you are nice to people, unless they are abnormal they will be nice too. everyone has their bad days too. live and let live. forget and forgive.
as for why people do not want you to take photos about their products even though they put it up on website, it's simple. people are control freaks that are inferior to your open-mindedness.
for example, if i am say, a very famous hk star (coughhackcough) who has many notches under his belt (literally).. i take photos of a certain object in my life which gives me great pleasure. maybe i show it to a lot of people, but you see, i would like to reserve the right that only i can take that certain object. if not will get death threats from all sorts of people, because they are unhappy with that particular object and the people i am showing it to.
ok that's not a very good example. in any case, it is their product, it is their shop, it is their property. if you owned an ice cream shop, and you felt that no one else should take photos of it except you.. would you be happy if people disobey these rules? i think the main reason why you are confused here (if you really are, instead of using logic to absolve yourself) is because there are not very clearcut laws in place with regards to protection of privacy/property rights. but why should that be your concern, are you very gian to take photos of people's things?
from what i remember, you can take photos of people's private property when you are not standing on the property, and it is in plain public sight and you do not have to resort to unorthodox means to photograph it. if you are ON their property, however, then it is a different story. but honestly, i think it is always good to avoid trouble, and respect people's wishes. sometimes all they want is to have a little authority, chat with them nicely and ask nicely again, i'm sure if you are friendly and respectful and explain your reasons something could be worked out, god knows it's worked for me a few times already.
if you ARE really interested, you can read more about something about this
here. there's a downloadable pdf file thing from some attorney dude that explains what people can do and cannot do with regards to your equipment and rulings under legal system.. although it is a us source though.
on another note, i wonder what happens if got say, a group of gangsters come up to confront you taking photos of them. i bet most of the people crying foul with regards to these security guard guys, police dudes preventing them from photographing stuff.. would just put down their camera and guai guai delete their photos without even a whimper. not that i'm saying that we should reduce ourselves to a primal level of violence, but it's just sad that we have double standards when it comes to the good guys versus the bad guys.