That's a great and practical question! From my observations and experience in various cities, the security of the ashtray on top of a public waste bin really depends on the specific design and installation.
Most commonly, these ashtrays are metal bowls or cylinders that are *meant* to be a permanent fixture. They are typically welded, bolted, or riveted directly to the bin's metal frame. In these standard cases, they are not designed to be easily removed by the public. You'd need serious tools like a grinder or wrench to detach them, making casual theft or vandalism difficult.
However, the key word is *designed*. Over time, bolts can rust and loosen, or welds can crack from stress or intentional force. In some older or cheaper models, the attachment might be less robust. While not "easily" removed with bare hands, a determined individual with some leverage might eventually loosen a poorly maintained unit.
The primary security concern is less about someone walking off with the entire ashtray and more about it being pried loose as an act of vandalism or due to lack of maintenance. Municipalities usually install them securely to prevent theft and ensure longevity.
So, to answer directly: Generally, they are quite secure and not easily removable under normal circumstances. But like any public fixture, their condition depends on the quality of installation and ongoing wear and tear. If you see one that is wobbly or loose, it's best to report it to local public works for repair, as a detached ashtray could pose a safety hazard.