Note added today: I found this bike while I was riding on the Somerville Community Path. The reason this is noteworthy is that another part of the docked bikeshare agreement signed by the four cities stipulates that no other bikeshare is authorized to operate in those cities.
This bike from VBikes, another bikeshare operator, is obviously in Somerville and is available for rental, and so according to many interpretations in violation of the agreement.
The issue is that this VBike is a dockless bike, and so it can be left by a user anywhere after that user is finished with rental.
The difference between docked and dockless bikeshares is pretty clear from the name. Docked bikeshare uses physical docks. You check out a bike from a dock when you want to ride it and must return it to one when you are finished. This makes for an orderly system with bikes in specified places, but it does so at increased cost: the docks are additional infrastructure, and docked bikeshare systems spend money rebalancing their bike inventory so that docks have some bikes but are not completely full (so that returning riders have an empty dock to return a bike to).
Dockless bikeshare has no docks but uses a lock integral to the bike which is controlled electronically, usually by smartphone app. A user downloads the app, which has a map showing the current location of bikes which are available to rent. When the user finds a bike they want to rent, they can unlock the bike with the app. They ride the bike to wherever their destination is, then lock the bike. The system then ends the rental.
Dockless bikeshare requires much less infrastructure but it also relies on renters to put their bikes somewhere reasonable at the end of the rental. This has often been a problem in other countries, including China, where the dockless bikeshare boom really took off. In general it's less of a problem when there are fewer bikes around, but because there aren't docks where you can reliably find a bike, it's important for convenience to have a number of bikes scattered around a neighborhood so people can find a bike when they need one.
VBikes and Ant Bikes (a company whose bikes I found in Harvard Square) are two dockless bikeshare operators in the Boston area who seem to have decided that the agreement between Blue Bikes and the four cities is something they're not going to worry about, and have placed bikes in those cities.
Other dockless bikeshare operators, including LimeBike, and ofo, have concentrated on towns outside of the four cities with an agreement with Blue Bikes. Last fall, I ran into bikes from both ofo and LimeBike in Malden. LimeBike in particular has indicated regions on the map in its app showing areas to which you are encouraged to return a bike and areas where you should not return bikes.
However, these areas are just a suggestion. No dockless bikeshare operator I'm aware of actually prevents you from locking a bike and thus ending its rental in a location they're not allowed to operate.
What is supposed to happen, and what the dockless bikeshare operators say they will do, is that they will collect bikes that have ended up in places they're not supposed to be and move them to places they belong.
In practice, this does not happen in any sort of timely manner. This is because the dockless bikeshare operators have all embraced a low-cost, minimal infrastructure model, and have few employees dedicated to moving bikes compared to docked bikeshare operators. Part of the point of the dockless bikeshare business model is to operate at lower cost and offer bicycle rental at a cheaper price.
So far, the bikeshare situation in Greater Boston has not gotten out of hand the way it has in various cities around the world, where bikes have ended up clogging popular locations, like the entrances to metro stations and other transit hubs, or in front of large buildings. But it is something to watch for.
The city of Boston has also begun impounding dockless bikes found in the city at a facility in South Boston, as they are in violation of their agreement with Blue Bikes. Where they will find additional labor to continue doing this and money to pay for that labor remains to be seen.