Hi, 

This is because of the privilege level 15 commands , which is by default enable on routers in gns3.

line con 0
 exec-timeout 0 0
 privilege level 15
 logging synchronous
 stopbits 1
 

If you want to restrict a router to login into user mode instead of enable mode, you have to run the following commands:-

router(config)#line con 0

router(config-line)#no privilege level 15

router(config-line)# exit

router(config)# do wr

Hope it helps.

Thanks