First off, how do you know that it was illegals who protested? If they would, CBP could easily pick them up. Besides, they can hardly afford to protest...
The protests are from legal immigrants and US citizens in support of illegals.
Now, while legalization of a certain kind of illegals may be unfair to the people who went through all the hoops to stay legal (like most here, me included), that doesn't change the fact that it is preferrable to get these people out the shadows. For example, here in CA, I pay extra for uninsured motorist insurance because illegals can't get car insurance (since they can't get a drivers license.)
If, for example, they could get a drivers license, there would be no need for the additional insurance.
And, this guest-worker program is anything but an "easy route," as everybody who actually reads through it would see.
Having said all that, I personally doubt that it is going to go through, since elections are coming up, and with such a highly emotional topic, no Congressperson wants to be seen as "soft on immigration."