Why did scholars say that doing the taslim with the imam is makrooh when the sahaba did it themselves?

As-salamu alaykum wa Rahmatullahi wa barakatoh. I came across a hadith which goes like this: > Sahih al-Bukhari 838 Narrated `Itban bin Malik: > > We prayed with the Prophet (ﷺ) and used to finish our prayer with the > Taslim along with him. And many people use this hadith as a evidence that doing the taslim with the imam is allowed and sunnah. Now my question is, how come scholars like Ibn Uthaymeen RH and others say that it is makrooh to say the taslim along with the imam when the companions did it themselves? Source: > Al-Nawawi (may Allah have mercy on him) said in al-Majmoo’ (3/464): > Al-Baghawi said: It is mustahabb not to start the salaam until the > imam has finished both taslims. This is the view of al-Shaafa’i > according to al-Buwayti, as was quoted by al-Baghawi. He said: Whoever > is (praying) behind an imam, when the imam has finished his salaam, he > should say salaam to his right and his left. End quote. > > Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said in al-Sharh > al-Mumti’ (4/267): The scholars said: It is makrooh to say the first > and second taslim with your imam, but if you say the first taslim > after his first taslim and the second taslim after his second taslim, > there is nothing wrong with that, but it is better not to say the > taslim until after he has said both taslims. End quote. So how came those Great scholars say that it is mustahab not to say the taslim with the imam, and that it is makrooh when it was a practice by the sahaba? JazakAllahu Khayran.