Imagine this: suppose there is a big company called Nile that sells a door security camera called Ding. This security camera happens to upload everything to the company servers. Since it is very popular, it has a huge coverage. If there are no restrictions on surveiling public ground, Nile could engage in all sorts of things, like tracking cars with license plates, etc. That's why Europe places restrictions on this sort of stuff.

You can still use a Ding, but it can only point at your private property and you have to inform people who may enter your property that it's being recorded.