Pheromosa is not currently banned from OU. Its suspect test is still ongoing, and is currently in the "voter identification phase" in which players can prove they have met the required COIL rating and number of games necessary to be able to vote. The thread says the deadline for this is 11:59 PM April 7th, so expect a verdict on the suspect test at some point after that time when the voting has taken place and a result is found. Basically, it will be over a week until we know if Pheromosa will be banned.
But if for argument's sake we assume Pheromosa will actually be banned in the suspect test, your contest as to why it shouldn't be doesn't make much sense. As mentioned by sumwun, if you're under the impression that tiers work such that if a Pokemon could be countered, then it is balanced, you've been misinformed. Mega Pinsir is definitely a hard counter to Pheromosa, but if it is the only good answer to Pheromosa, then Pheromosa is certainly not balanced because then it forces everyone to use Mega Pinsir or else you will lose to it. That is not in the interest of the game at all; anything that applies such a strain to teambuilding is overcentralising and unbalanced. Mega Pinsir isn't the only way of dealing with Pheromosa obviously, but this principle certainly manifests in all of Smogon's decision making and debates.
As for why the suspect was held and why Pheromosa might in fact end up with a ban, the suspect announcement thread here explains it quite well, as do actual Smogon OU council members in the post following. Basically:
- Pheromosa's raw Speed stat is the highest in OU. As a result of that, it is perhaps the most difficult Pokemon in the game to revenge kill. The popular Choice Scarf set cannot be outpaced unless you have a moderately fast Pokemon at +2 or priority. Beast Boost makes this even more complicated, as does U-Turn.
- Pheromosa's offensive stats are also extremely high, giving its High Jump Kick and Focus Blast extreme offensive presence and making Pheromosa a difficult Pokemon to switch into.
- Pheromosa has a very good movepool to take advantage of its two high offensive stats. Though its STAB attacks give poor coverage in general and would be walled with relative ease, its access to moves like Poison Jab and Ice Beam mean Pokemon that would otherwise check Pheromosa cannot (e.g. Landorus-Therian who would be huge against Pheromosa if didn't have to contend with Ice Beam). It even gets Rapid Spin.
- Pheromosa confines teambuilding, as mentioned above. Only very select types of Pokemon can reliably switch in to Pheromosa without knowing its set. Again with your Mega Pinsir example, Quick Attack is a play you could make after Pheromosa knocks out a Pokemon, because you can't switch Pinsir into Pheromosa because your opponent may predict you and choose a different move. In terms of Pokemon that do switch into Pheromosa, you are generally limited to defensive Ghost- or Poison-type Pokemon that aren't weak to Ice Beam, and some select others; otherwise you have to make risky predictions in order to take the hit. Toxapex, Alolan Marowak, Mega Venusaur, Mega Sableye, Mantine and maybe Pelipper are the only viable Pokemon that can switch into Pheromosa with absolutely no fear that the opponent's moveset could pack a nasty surprise for them. Seems a decent amount, but if you have to use one of these to not have a Pheromosa weakness, the Pokemon is still overcentralising and stagnates team diversity.
- Pheromosa is unpredictable. Smogon doesn't like Pokemon that are so versatile that each of their different sets have different forms of couterplay. Though you might be able to pick a different Choice Scarf user on your opponent's team, there's no way to tell for sure if it's running Scarf, Focus Sash, Life Orb, Specs, Fightinium Z or even something else. This means it is a hard Pokemon to actively counterplay until its set is revealed to you in some way, at which point the damage has probably already been done. People can even modify Pheromosa's EV spread to get a different type of Beast Boost.
As far as I can tell looking through the discussion thread and the opinions of council members (whose votes are weighted extra iirc) there's a strong chance Pheromosa will end up with the ban.