1. If the Trump campaign colluded with the Russian government's covert operations to influence the American election that would amount to treason. 
2. Flynn could be innocent of anything illegal, sure. Maybe he's not. Depending on the outcome and the additional information found out it could potentially indict president Trump if he knew of anything illegal Flynn had done and then actively tried to block the investigation because of it. At the very least for a sitting president to take the actions he did shows hypocrisy, dishonesty, and poor judgement. It's indefensible on principle.
3. In one breath you say the constitution is important to uphold, and then in the next breath you decide to turn a blind eye to a foreign government actively trying to sway our democracy (collusion or not). The cognitive dissonance is overwhelming. 

The partisan hypocrisy alone is reason for the distaste, with our without legalities.