That's not how I read it.

> the contribution back part is not mandatory

In that case weak copyleft would not be any different from MIT/BSD.

First, assuming you are distributing only the larger work:

"3.3. Distribution of a Larger Work

You may create and distribute a Larger Work under terms of Your choice, provided that You also comply with the requirements of this License for the Covered Software. [...]"

Assuming the covered (OSS) work is distributed as an executable:

"3.2. Distribution of Executable Form

If You distribute Covered Software in Executable Form then: (a) such Covered Software must also be made available in Source Code Form, as described in Section 3.1 [...]"

Finally, 3.1:

"3.1. Distribution of Source Form

All distribution of Covered Software in Source Code Form, including any Modifications that You create or to which You contribute, must be under the terms of this License."

The only wiggle room I see is not triggering the distribution clause by a SaaS-only offering. By the way, EUPL is a non-viral copyleft license that closes the SaaS loophole in MPL/EPL/LGPL.