Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

1.6.3
2. What operating system and processor architecture are you using (go env)?
OS X El Capitan 10.11.6

1. Issue
   The "default" label in a struct does not imply "optional" tag.
   According to A Layman's Guide to a Subset of ASN.1, BER, and DER
   http://luca.ntop.org/Teaching/Appunti/asn1.html
   Section 5.12 SEQUENCE "The OPTIONAL qualifier indicates that the value of a component is optional and need not be present in the sequence. The DEFAULT qualifier also indicates that the value of a component is optional, and assigns a default value to the component when the component is absent."

The default tag should imply optional
This problem is recreated using code snippet below

package main
import (
  "fmt"
  "encoding/asn1"
)
type test struct {
  A int
  B int `asn1:"default:1,tag:1"`
}

func main() {
  e, _ := asn1.Marshal(test{A:5})
  fmt.Println(e)
}

$ go build && ./test
[48 6 2 1 5 129 1 0]

As you can see, asn1 marshaled/encoded 0, which is the default for B in struct test.
While compliant implementation should yield
[48 6 2 1 5] leaving the B part out.