Does that seems a museum? That's the problem with Venice (stupid) tourists, they think it's a open air museum, it's not, it still is a city with people living there.
To be fair, that totally looks like a tourist attraction to me. Where I come from the only places painted in bright colours like that are theme parks or children's play grounds. No "real" housing would be painted like that, so the assumption may have been that these must be for tourists because why else would they look like that? Of course different cultures are different though (this is why we travel in the first place - to see and experience something different) and things will therefore be different than they are back home, so people should check their assumptions when traveling.
I guess what I am getting at is that people's cultural upbringing is different, and so their expectations are different. This is unconscious bias but for houses I guess. That is not to excuse people for entering someone's home without permission, but that does look like an attraction to me. Personally I would not just walk in unless it was obvious, but then everyone is different.
People should learn something before visiting a foreign country, especially a unique city like real Venice, not a cheap copy in Las Vegas.
Or Gran Canaria, or Netherlands, Spain, Austria, Norway, Singapore, Mexico, Brasil...
World cultures ignorance is staggering in some Anglo-Saxon countries...
Locks have been a thing for a long time so this is not exactly a new problem.