You are so right. I found that while I was teaching in China, I spent a lot of time undoing the bad teaching, so I could teach them correctly. I did meet many teachers in China that had excellent American accents, and so they got the best jobs in the best places. Unfortunately, many Chinese parents who do not speak English cannot tell the difference between a teacher with good pronunciation and a teacher that does not have good pronunciation. 

As to the "th" and the "s" here is a funny video about that...
<a href="http://www.stupidvideos.com/video/commercials/German_Coast_Guard_2/#4052">www.stupidvideos.com/video/commercials/German_Coast_Guard_2/#4052</a>

People always try to find some similar sounds from their first languages to replace the pronunciations of the languages that they're learning, that's the problem.

Take Chinese as an example, we don't have a similar sound for "th", so we tend to deem "th" the same with "s", that's the horrible part.

That's partially why people get accents - they always believe like Chinese, we're confident to see we got the most sounds already in Chinese. Some inaccuracies of some particular sounds are no problem in my opinion, but speaking of accents...

It took me about 3 years of confusion and intense research to learn it well. There is a lot of confusion out there over the differences between British English and American English phonetic sounds and symbols. I have even seen some texts books write using the British English IPA symbols, but the audio for the text used an American accent - great way to confuse students. 

However, I have found the IPA to be a very effective tool in the classroom. For those living in Asian countries that start to learn the IPA at a young age, I can use it to quickly help a student pronounce a word correctly. I can show them both how they are saying the word incorrectly and also how the word should be said. 

For example, in China students have a hard time saying the word "moon." They will say /muən/ instead of the correct way /mun/. Also take "usually," they will say /ju ər li/ instead of the correct /ju ʒu əl i/.

Didn't know that; 45 vs 48, I thought they were the same.

We don't quite emphasize B E or A E here when we start to learn, we just learn.

But still surprised sir you are one of the very rare E speakers who read soundmarks. You a professional in this field? Phonetics is quite interesting.

Also study the differences between [e] and [ɛ]. You might find the study interesting.