People are missing the point with regards to the Japanese tanker. Read Nathalie Tocci's comments about this (say, here).

There is a delicate balance in the Persian Gulf between Iran and Iran-backed forces and the opposing side. The modus operandi is always of measured response. If Iran perceives an escalation, they respond in kind. If they don't, the escalation will follow another, and soon the balance changes. These tit-for-tat games are how the situation remains stable.

If Iran is indeed behind those tanker attacks, and it's very possible (though proof is so far lacking), then it is a signal that they consier the recent pressures, force mobilization, extra sanctions, bellicose statements, etc, as an escalation, and a deviation from the norm.

This is not the view you hear in the press, because it's too big a dose of Realpolitik to say it's normal to use measured violence as a signal, but this is absolutely how intelligence experts are going to interpret this. It would also be the reason why the boats were not sunk, because that would itself be another escalation, and not a response in kind.

To understand what's happening, you have to ask what has changed recently to warrant this kind of signal from the Iranians?