It doesn’t take a 10 to make a review an outlier when a game gets such a low score (e.g. SoT). That Xbox score is 8 points higher than any other score and 19 points higher than the average (which will surely drop several more points, likely down to around 66). 

MC gives different weights to different sites. So random fan site A B or C rating a game 15 points higher than the average, won’t have the effect assumed. Especially if that website has a history of “inflating” review scores. Also, until all review sites use the same scoring method, sites like MC and OC are flawed and should not be taken seriously.

To use the OPs example game, if I were to review SoT right now I’d say 7/10. Which converts to a 70 on MC. But if I were using a 5 point scale, I’d say it’s about a 2.5 out of 5. To me, that’s right about at a 7/10 game. But in MC, it translates literally, which means 50. But what if I use a letter system? I’d say SoT is a good C game right now. A C is not bad. I’d love a C in some of my college classes right now. But on MC a C is a 50. There are just too many different scoring systems with their own interpretations to take these aggregate sites seriously.

8 out of 18 Reviewers gave the game a higher score than average yet you are going to complain about the Xbox bias sites? I personally played the game and I think the game is an 8/10 and I can see other sites loving the game while others will hate it. I don't see your point here. You want them to give the game a lower score on purpose? Isn't that what opinions are for? Maybe those Xbox sites love Sea of Thieves just like the other 6 sites rating it higher than average.

You can find biased reviewers on any gaming related website. Instead of just looking at the Metacritic score and judging by itself, people should rather actually read the reviews to see if their score is justified and if they make good points.

Yep. This is why I use Opencritic. I can use my own personal settings to "ban" clickbait sites like Slant, and Metro. 

This is why I generally only accept reviewers that work off a five or ten point scale on Opencritic. The sites that work off a five point scale have to use half stars in their system. 

IMO one huge flaw with Metacritic is that they give weight not based on current predicting merit, but based on what is sometimes a twenty year history of review scores. For example according to Metacritic Gamespot is still a top tier site, because it has been around for twenty years. Same thing goes for IGN. A crappy review outlet can turn into a good review outlet in five years time, and vice versa. But Metacritic isn't set up to handle this. 

But anyway, to answer the question in the OP....

Yes, they should be allowed. But they should not be given credit unless their five year history shows that they are generally unbaised. So if their reviews for games are always higher, then they shouldn't count as much towards the average. 

Please tell me why 7/10 translates to 2.5 instead of 3.5 on a five point scale. Don't take this question as anything aggro. You've filled me with wonder. 

What I meant was that they didn't inflate the score just because it's a Nintendo game.

A review is a review. It's not a competition. I know some companies used to (and maybe still do) base their employee bonus on Metacritic scores but that ain't got nothing to do with me.

All I care about is if a game is worth buying based on my particular tastes. The final score doesn't mean a thing to me. All I care about is the details. I think we gamers place too much emphasis on the final score rather than the content of the review. Do I care if Madden 2019 got a 99/100? Not at all because that's not my type of game. Do I care if WWE 2K19 got a 69/100? I'd still buy it but the reason why it scored so low is something I'd be concerned about.

It is hard to say because they usually review a bunch of other games on the same platform aswell (non-exclusive games) but when it comes to their reviews of exclusive games they may seem a bit biased but at the same time reviewers can have a preference for a type of video game aswell and maybe only one platform provide those types of games who knows?.
Also some platforms have more exclusive sites than others so they may get an unfair coverage and lastly just because they are a site dedicated exclusively to a platform it doesn't mean that they are giving them highly favorable reviews. Just look a Push Square. A playstation centered website who does a ton of PSVR reviews but more often than not their reviews are actually lower than the average for the PSVR exclusive games etc.
So it is quite complicated and there are a lot of parameters to consider.

Yes, but only if Metacritic are renamed to Pseudocritic or something like that