Fasting On A Monday

Hadrat Sayyiduna Abu Qatada al-Ansari (Radi Allahu Ta'ala Anhu) reported that Allah's Messenger (SallAllahu Alayhi wa Aalihi wa Sallam) was asked about fasting on Monday, whereupon he said:

"It is (the day) when I was born and revelation was sent down to me."

References:

1. Sahih Muslim, Book of Fasting, Vol. 7, Page 323, Hadith 2807

2. Sunan al-Kubra lil Bayhaqi, Vol. 4, Page 286

3. Musannaf Abd ar-Razzaq, Vol. 4, Page 296, Hadith 7865

4. Sunan Abi Dawood, Vol. 7, Page 255, Hadith 2428

5. Musnad Ahmad, Vol. 49, Page 195, Hadith 23200

6. Usdul Gaabba fi Ma'arifat as-Sahaba, Vol. 1, Page 21-22, Published in Lahore 1987.

A simple question to the opponents:

When the beloved Prophet (SallAllahu Alayhi wa Aalihi wa Sallam) is celebrating his birthday not yearly but every Monday then how it can be labeled as Shirk or Bid'ah?