views differ for various reasons, discussed here.

Baltic Times
NEWS
Treason: the real cost of Simm’s betrayal
Oct 01, 2008
....
TALLINN - While some fear Estonia’s reputation as a NATO partner has been tarnished and others believe it’s stronger, most agree that national defense has been severely compromised by a deviant individual and some suspect widespread espionage. But where exactly does Estonia stand after Herman Simm’s treason?

Ask any Estonian whether Russia should be perceived as a threat and the answer is nearly always the same: Yes. The notion that history repeats itself seems deeply ingrained in the thoughts of Baltic citizens, and given the lengthy and disastrous consequences of the Soviet occupation, this hardly comes as a surprise.

With this in mind, it’s no wonder that the nation has bestowed upon Simm the status of public enemy number one. There’s nothing redeemable about selling state secrets to a nation’s long-term oppressor, especially when residual skepticism of Russia has existed in the Baltic states since their independence in the early ’90s.

Even more alarming, a British former civil servant has informed The Baltic Times that this might not be a one-off case, but part of a wider breach of national security. .....
http://www.baltictimes.com/news/articles/21475/

The 3-15 year sentence range for treason and espionage (noted in OP and in above article) seems a bit light - no.