It's a should, not a shall. So the installation isn't illegal, but it is poor practice. The engineer who drew up that ramp interface didn't give a shit about best practice. Flexibility is allowed to accommodate cost issues (ie drainage), but thats not the case here.
We should expect better in a walking city in 2018
How else would you align it though? At the last minute, the City had them redesign the crosswalk and add a curb extension at Charlesgate West. The dome panels they ended up building would have lined up with the old poorly-placed crosswalk in the original proposal. I think we ended up with a better project in the end, especially for walking. You no longer have to walk 120ft down a side street to stay on Beacon St.
The curb extension for the plaza on Beacon itself was cut back in anticipation of continuing the parking protected bike lane further down towards Kenmore Sq.
See page 12 for the original proposal: http://www.bostonplans.org/getattachment/f1ca271b-b701-4c78-92e2-4bcded3f0bfe