For those who can still think by themselves without MSM guidance here are some facts:
1) Iranian oil exports were never under UN sanctions. And in 2016 all other UN sanctions were lifted up.
2) Syria is not under any UN sanctions.
Now, for those who don't understand difference between UN and unilateral sanctions:
UN sanctions are the only ones that can be (however remotely) considered as lawfully obligating for the country under sanctions. Unilateral sanctions are applicable only to the citizens and companies of countries that instituted them. For example, if country A issues unilateral sanctions against country B, it has jurisdiction only over citizens and companies of country A. It can send them to jail, confiscate their properties etc. But, if country C conducts any business with country B, then country A can do nothing. Well, lawfully, of course
So, the way things stand right now:
1) UK seizes Iranian tanker going to, as UK claims, to Syria. Since UK did not declare state of war with neither Iran or Syria, this act is qualified as piracy by international law. Why? Because there is no UN sanctions that could prevent such operation, and no unilateral sanctions can be applied in this case. Read this again, until you understand. To be clear - if tanker would have been flying any EU, or UK, or US flag, only then unilateral sanctions could have been applied. Since this was not the case, this seizing is an open act of piracy.
2) From now on, since act of piracy was conducted by a vessel flying UK flag, and UK govt openly admitted that it authorized the seizing, this makes - according to international law - any UK-flag bearing vessel a pirate vessel. I.e. every country on the planet now has a right to confiscate UK-flag bearing vessel and hang all crew members.
By the way, this applies only to vessels flying UK flag, not the vessels that are owned by UK companies. Someone in Tehran forgot about this nuance at first, so first they seized tanker owned by UK company, but flying Sweden flag. Then they corrected themselves - the did let go first tanker, but the second one is detained absolutely in accordance to international law, since it was flying UK flag.
But then why UK did it in a first place? Well, let's put aside the reason that UK was always a pirate state, and look at the economic reasons - the only ones that really spin the world.
Many did hear about shale oil. Let's put aside all the environmental damage that occurs from it's production - oil companies never cared about that. But what does matter is that to be marginally profitable, oil cost should be over $80/bar - that's the self-cost of shale oil. But as you know, for last few years crude oil prices were hovering in $50-60/bar range. Western oil companies that ventured shale oil drilling accumulated huge debt. They are on the brink of collapse, and would not survive past the end of this year - all their credit options are already exhausted by now. So, the goal of US and UK is to move crude oil prices to a $100-$120/bar. Price over that range will destroy US and UK economy, but current price will destroy US and UK oil companies. Question is, how the target price can be achieved? It is estimated that full-blown war with Iran will send prices over $300/bar. So, it's not an option. But, some short-term assault on Iran with careful destruction of its oil fields will move oil price just the right range. That's why US and UK are actively trying to create casus belli for such assault. So far it doesn't work since Iran stays within international law bounds.
I'm pretty sure we soon will see some more US/UK provocations.